kitten not growing like his brother

One of the other foster moms has two orange and white boys who are about four months old now. One is much smaller than the other. Perhaps he was the runt, but he also had worms bad and diarrhea.

She is concerned and wants him to gain a little weight. He is quite skinny.
He was on Royal Canin Recovery - I think she just got another flat of it. (Another cat is eating it ,too. ) But is there any thing else she can do to help him out? I was thinking maybe a little stage two KMR?

What foods would be good for him to help him reach optimal growth and development? She is on a limited budget and the rescue is really strapped for money.

His name is Cooper. I am sorry but I don't have any pics of him. Maybe I can get some. He's adorable!!

I have a very skinny stray that I am feeding and I toss all her food with powdered KMR. It seems to be working as she is fattening up a bit.

Some kittens are just smaller than others. It's normal. However, if she wants to fatten him up I would recommend getting some KMR and let him drink it from a bowl if he likes it. Otherwise you can mix it with canned food. I would also make sure that he always has access to dry food and gets canned food too a couple of times a day.
Nutrical added to the food or given directly would also help. It's high in calories which is what he needs. Baby food is another good option that she can supplement the kitten with.

Good luck

I usually use Nutrical or similar supplement.
Another thing I got from my vet when I had a skinny, wormy feral rescue kitten is something called VAL Syrup. The kitten loved it, I put it in an oral syringe and he would lick it up.