Hi Jean,
Welcome to TDKland. We’re the furriest forum around. Thanks for helping mum out with her kits. Here you thought she was just putting on weight. 😉
I don’t know the efficacy of Royal Canin kitten formula but, here in TDKland, we recommend Goats’ milk for kittens. Cats are lactose intolerant and can’t digest cows milk. Goats’ milk is full of nutrients, probiotics and gentler on their tummies. This can also regulate their poop. If you are using the Goats’ milk concentrate, mix it with pedialyte. Pedialyte gives an extra the extra boost. Keep in mind if they seem lethargic put a dab of Karo syrup or honey in their mouth. Sometimes this is all they need to give them the added energy boost to get them eating. A smidgen of mineral oil in their formula can usually help them poop.
He’s the runt and may not have figured out the nipple. So try using a syringe. It will take longer to feed him but he’ll get fed.
Here is a link with good information on caring for your wee kits.
http://messybeast.com/handrear.htm
Good Luck
PG