Weaning questiions and concerns

Evening all!

The little kitten my husband and I rescued 3 weeks ago is growing fast and thriving! We are so happy with him, but we believe he's coming up on his 4th week and we think it's time to introduce him to his litter box and weaning him off the bottle. Today was the first day for both. He actually did well with the litter box, and he immediately started digging, but still needs some help in the "voiding" department

However, he's really not interested in taking any formula from our fingers. He'll sniff at the formula we put in a jar lid, but will just walk away from it. We know it's going to take some time, but the only reason we're concerned is that he doesn't seem too interested in the bottle anymore either. When he wakes up he will act hungry, but he'll just chew on the nipple of the bottle. Eventually he'll drink from the bottle, but definitely not as much as he used to. We thought maybe he was starting to get sick, but he's not only acting normal, but he's becoming more and more playful when he's awake. We've even graduated him from a little box to his own cat bed which he goes in and out of no problem to sleep.

What's going on here? Do we just not need to feed him as much anymore?

I know I sound a bit worrisome, but I just don't want my baby to go hungry or unhappy with us

Are you weighing him at the same time each day?  Stopping gaining weight is a very good clue that all is not well.

As you know weaning for kittens with their mother takes some time, and they tend to wean themselves by exploring their food (walking into it!) and by watching their mother.  Obviously he can't do the second, but if you put a plate down with a tiny bit of smelly food he will probably start exploring it.  I would start weighing him in grams (you need that level of accuracy and using them avoids decimal points) to see how his weight is going.