Teen kitten not as playful and a little skinnier than usual.

So i had a litter of three kittens about three or four months ago they were all very healthy and doing well. Just recently one of the kittens looks a bit skinnier than usual. Its not a huge weight drop at all just a little bit. He hasnt always been the most playful but he's not near as playful as he used to be he tends to sleep , relax , lay on the bed , cuddle and purr most of his time. I may just be over reacting but I'd like to know for sure.

Have there been any changes in the kitten's eating? Have the kittens been dewormed or been seen by a vet? Sometimes kittens will grow more slowly or put on less weight if they are not dewormed. Have you noticed if the kitten is teething? Some kittens will eat less while they are teething because their gums hurts.

Do all of the kittens eat at the same time and have access to the same food? Sometimes this is more of an issue than anything else with kittens especially as they get older. You might want to watch and see, maybe other kittens are eating more and this one isn't getting enough? This was one of the biggest challenges I had with kittens and sometimes they had to be fed separately.

Detroitcatlady has lots of good advice and speculation!

When you say "teen", I assume older than 6 or 8 months?

They all lose that little pot belly as they get older. 

I have similar issues with my kitty actually. He is 11 months. He has always been on the lanky side. His vet said this was fine cause it meant his body was putting the nutrients towards bones and such. He is still rather petite and I honestly do not think he has gotten much bigger in a month or two. I am only now starting to notice a bit of mass gain. He is healthy as far as I am aware, and eats plenty of food. He recently appeared to get a bit skinnier again as well.

How often are you feeding them? Free range would be acceptable still if he isn't quite ready for set meal times.