Trying to trap a feral cat that's stopped eating, at same time as a mom+kits.

Background:
When my SO and I lived with his parents, I started caring for a feral black cat I named Salem. He was a BIG boy. Not fat, but bulky.
I easily (like, got her to come to me and picked her up) caught his pregnant lady friend two days before she had her babies. I actually kept her and the babes.
Salem, however, genuinely acts like a feral cat.
There was also a tortie girl in the area. I called her Cala. Cala doesn't seem feral per say - she's quite vocal - but she wasn't about to come near me willingly either. Cala got pregnant around the time we moved out of SO's parents house. She now has 4 kits I'm guestimating at 2 months old tops, but I'm not sure. I've only seen them twice but they're still quite small.

Now:
We convinced his mom, who started feeding them in my place (but feeding them way way more, so now there's probably a good chance these kits won't know how to take care of themselves if something happened...) to finally buy a trap. If we catch anybody, they'll come to SO and I's home and be put in the spare room until we can find a shelter to take them, since before, she called asking for help and was told they're full.

The problem is, Salem stopped eating at some point. He went from being big and bulky to being THIN. He also seems to have some sort of wound/hair loss on his back. Mom cat Cala also may have some hair loss, so SO's mom has said.

At some point, ALL the cats vanished for ~4 days and since they've come back, Salem hasn't been acting right. Spooked, won't eat, scrawny.

So I'm hoping that mom + kits will eventually fall for the trap, which I've baited with stinky foods. Get them taken care of and rehomed.

Although the other day the food was gone but the trap was empty (we tested it, it works) so... we're not sure what happened there.

But how am I supposed to lure Salem when he won't eat?
He actually appeared minutes after we put the food out in the trap, but he has no interest in it. He just laid down nearby and that was it. At this point we're all pretty worried about the guy. He's stuck around for almost a year now. I just want to get him taken care of... and fixed.

I am sorry about poor Salem.  It appears he may be quite ill.  Since he is not neutered it is very common for him to wander miles and miles in search of a mate.  During the wandering is will get into fights with other tom cats over the females as well as over territory.  The fights can be quite vicious.  There are numerous illnesses he can get from the fights.  The wound on his back could be infected and he can be quite sick.

Trapping would appear to be very difficult since he is hardly eating.  You might try getting the stinkiest food you can find such as tuna in oil, sardines and some people use a chicken leg from KFC.  They tie it to the top of the trap and let it dangle in the back.  The problem then is removing it.  If he still refuses to eat anything, you can either try to get near him and throw a heavy blanket over him and scoop him up and into a carrier.  YOu would need to wear thick gloves and long sleeves.  They also sell nets that can be used.

I hope that you are able to find a way to help him.

It is a must that all of the other cats get trapped.  Have you ever trapped before?  The key is to not feed them for a day.  Then they are all extra hungry.  Here is a great video on trapping tips

That's my concern about Salem. If he is ill, I'm hoping there's nothing wrong with him he could pass on to the mom cat Cala and their four kits. Pre-weight loss, when their kits were still tiny,  he was apparently already at, er, attempting to make more. 

Right now I am using tuna in the trap (it's also what he ignored) but if that doesn't work I'll keep the other two in mind. The biggest issue is that I no longer live at the home he's being fed at, and while SO's mom is happy to feed them and care about them from a distance, she doesn't want to get much more involved than that.
He does often lounge on the porch of the (abandoned) house next door but when the front door to my SO's folk's house opens, he immediately tenses and bolts if you approach. At least he's still active I guess.

I agree about the cats though. Two adults plus four kittens... It could become a problem fast. It's been two days and we haven't caught anyone, despite the instance of food-gone-but-trap-not-triggered.  I'm hoping SO's older brother isn't tampering with it since he's against us trapping them since he enjoys watching them since... he has nothing better to do.
When I was over there last night they'd just stopped putting out wet food but still had kibble out. I shut that down however so maybe I'll have better results in the next day or two.

I haven't used the trapping method before but I've been researching it, so thank you for the video. I'll try adding a blanket to drape over the trap.