Urgent!! Kitten not moving much.

I’ve been taking care of 2 kittens for about a month and few days. Recently one has seems a little sluggish like she hasn’t been feeling well. The other was fine except he’s not been eating as much as he normally does. I went to go get him for his next feeding and he was just laying there. I thought he was dead because normally when I go to get them to feed he is the first one to jump up, but he was stiff and barely breathing. My fiance’ went and got some syrup and I gave him some in case it was low blood sugar. He was sucking on my finger(very strongly) so I hope that’s a good sign..but he isn’t moving very much. I fed him and he ate a good bit, but I don’t know what else to do. I have him wrapped up in a blanket beside me keeping him warm and checking to make sure he’s breathing every few minutes.