Another thing you can do if he starts chewing on your hands/feet is a kind of high pitched "Ow!" or some similar sound (not loud, just higher pitched). It's close to the sound his littermates would make if he were playing too rough with them. That's the sound I make when my fosters get too excited about my feet. Or knees. Or back.
But given his age, I wouldn't expect him to instantly get what you're trying to tell him. It'll take a few weeks.
Kittens should gain about 4 ounces a week, so if he's 3 weeks old, he's right on track for weight. If he's 4 weeks old, he's a little on the small side.
At 4 weeks (which he may or may not already be) you can try to get him interested in solid foods. I always have kitten crunchies out for my fosters...and then they can nibble if/when they decide they want to. Human baby food (meat flavors like ham or turkey) is how I tempt my little ones. Ham especially seems to be a favorite. He will still need formula for a few weeks, but once he figures out solid food, that might ease your feeding schedule a bit.
Just put a tiny bit of the baby food on a spoon. If he doesn't take a bite/lick, you can put a tiny bit in his mouth (not enough to choke him--more like a little taste). Once they swallow that tiny bit, they'll either look confused or get really excited about the spoon. With my current fosters, little girl kitten figured out baby food after two attempts with the spoon. Her brother took over a week.
Once they've figured out that solid foods like baby food are yummy, you can start slowly adding in wet kitten food and get him used to that flavor. Then just decrease the amount of baby food in the mix slowly over a few days or a week until he's eating straight kitten food.
Raising kittens is a lot of work, but at 4 weeks he's through some of the toughest parts already. Good luck!