Bless you for (properly) caring for these cats!
I'd normally caution that cats that have lived outdoors for most of the time have trouble adjusting to being indoors 100% of the time, and in some instances it's just kind of cruel. As the others have pointed out, it really depends on the personality of the cat.
But in this intstance, it seems to me you really have to try, given that you may have to move overseas. I realize its three years from now, but you'll need the time for him to be able to travel and adjust if you are going to make that move overseas.
What I haven't seen mentioned yet is what I think you're going to need to give you and DaddyBoy the best chance at making this work, and that's a safe room. If he's going to bonk out and shred, scratch, and spray when he realizes he can't get out... you might want to consider containing it to just one room for that first few weeks (or more?).
I'd buy a Feliway plug-in and plug it in for two days before you ... lock him in there. I second the suggestion of using Cat Attract litter, and definitely provide at least two boxes for him (he's not used to peeing and pooping in the same place).
If you have a tall cat tree, put it in there. If you have a bird feeder, put it out the window so he's got "cat TV." I know he's not used to toys... you might want to start introducing some wand toys so he knows what they are and they don't just freak him out, because he's going to need play interaction.
I LOVE the idea of a stump or sturdy branch for him to scratch.
Sorry for the length, but maybe something in here will help you decide how to proceed.
Now... all of that said... we have 8 cats, all feral rescues. One was a year old when we brought him inside full time in 2003 (Tuxedo) (but he'd known us since he was 4 or 5 weeks old), one was about 2 when we brought him inside in 2008 (Billy) (and he'd only known us for the summer and part of the fall); the last one was either 3 or 4 and we didn't know him at all when we brought him inside (Chumley, last year).
We live in an RV. We have no way to separate cats for socialization or proper introductions.
Tuxedo. We already had three of his siblings inside full time as pets, and one adopted out. He was unadoptable. Mean to us, TERRIBLE with other cats (sent two to the hospital needing stitches). His switch flipped that first winter, he headbumped my husband, and that was that. But we couldn't risk him with the other cats, so outside he stayed. The temps fell to sub zero (actual, not windchill) late that February. My husband couldn't take it. Tuxie wasn't using any of the shelters we put out for him. We found a (new) empty boarding facility willing to take him (for shelter only). Inside he went. We visited him frequently, he adjusted.
He also developed a UTI, went to the hospital, stopped eating... and the vet told us we had to take him home, he was just dying there. So we did. And poor little guy was so weak, he had no fight in him, and moving in full time went without hitch.
Billy. He and his brother were born feral, we know this from the neighbor in the RV park that was caring for him and his brother (feeding them). They were just ... "raised" around friendly people. We got them TNRd, and the guy that was taking care of them left.
We couldn't foster, but we tried to find them a home together. We found a wonderful home for one of them (HATED to split them up), and it was BREAKING OUR HEARTS to have Billy out there alone. We simply couldn't take it. But Tuxedo (different story) has a compromised immune system. So we could not bring Billy inside without being quarantined at the vet to make sure he wasn't bringing anything in here with him. Turns out he had giardia (no wonder we couldn't get him to bulk up for winter). He was terrified at the vet for two weeks. But when we released him into our bedroom... well, our theory is that he was just SO RELIEVED to be out of the vet boarding, that he simply accepted our home.
Obviously he'd learned to use the litter box at the vet... so we just had no issues with him.
Chumley. He was so food aggressive when he turned up here, all the other ferals disappeared. He was also so food aggressive, he didn't run from us. We didn't know what to do with him, but we couldn't release him back outside after trapping him for his neuter. So we rented a trailer, parked it in the site next to us, and released him there. We figured we'd give it a shot at fostering him, and get him in the foster network and up for adoption. He did not know what the litter box was. When he first peed, we soaked up his pee and put the paper towels in one box. When he first pooped, we used paper towels to put it in the other box. That was that, he used the boxes from there on out. Chum's story was different than your boy's. Chum had a formerly broken leg that healed on its own, he was very beat up, parasite-ridden, had obviously been very hungry, had an abscess on his foot, and thus it shouldn't have been a real surprise to find out he was FIV+, and he was in bad shape. Horrible diarrhea, allergies, lots of immune-related problems. So as it turns out, he was just grateful to be safe and have food. He lived in the trailer on his own (obviously we spent a lot of time over there) while we socialized him. After two months we began slow introductions to our gang in the RV... that's a different story. But he moved over herefull time in August (after being released into the trailer in May).
Your boy is more akin to Billy than Tuxie or Chumley. The difference is that Billy had never been indoors before. We fully believe what made his transition ... "easy" ... was that he spent two weeks at the vet. I think that was so terrifying that almost ANY alternative was better than that. Now... as DaddyBoy is not ill, if you want to consider the "shock" approach to moving him indoors, you could consider boarding him at the vet for a week or something. He may be so much happier in a safe room, and so grateful for "being rescued" from the vet, that he transitions much more easily than he would otherwise. It seems a harsh (and expensive) way to go about it. BUT... it may also help the transition go that much more quickly. So yes, far more traumatic in the short run... but potentially worth it in the long run.
We had no choice because of the other cats. But it did produce unexpected results. So just putting it out there.
Vibes and hugs to you on your journey with DaddyBoy!