Flat chested Kitten?

My 7 kittens were born March 3 and are 17days old. One day when I was changing the towel in the nest I only counted 6 kittens I frantically searched and found the littlest under  the momma this happened when they were about 4 or 5 days old.When they were about a week old I noticed the runt of the litter when I picked him up his ribcage was flat or kind of squared. I read up and found information about splinting and tried it.When I applied it he immediately started crying louder and breathing improved.It doesn't impair suckling or movement at all.He doesn't even seem distressed by it either.He nurses longer too I have noticed. If you have done this or had kittens with this could you answer... How long do you leave it on for?  Did the kitten survive or grow out of the flat chest with out the aid of a splint?

I did have one flat-chested kitten, but she was born that way.  She was shaped like a turtle and did have the "pectus excavatum" or caved in chest deformity.  Because of that, mine was not suitable for splinting as it would have killed her..  The advice was to just wait and see, that if she made it to 3 weeks without dying, there was a good chance she would lead anormal long healthy life.  We named her "turtle" and just watched her.  She hung in there just fine although smaller than the rest of her litter.  Mamahad more milk than she needed so she was never short of food.  She was also what is known as a "swimmer" which means she could not get her legs under her to walk as soon as the other kittens did, so she kind of swam around the next.  However, she continued to grow stronger and improve.  We would help her learn to walk by just lifting her up a little so her legs could actually get underneath her instead of sticking out to the sides.  Eventually she learned to walk, run and play and was adopted into a loving home along with her sister.  As far as I know, she is still fine.

his mother does not want it on him she removes it constantly, so I have considered taking it off and just letting nature its course or just leaving it on for a few hours at a time

From what you say, perhaps it's most useful to use the splint when he's nursing and then take it off after that since mama is going to remove it anyhow.  Mother cats often know better than we do, even when it breaks our heart because they will abandon a sick kitten and move the others elsewhere.  I saved one of two who were abandoned like that (got there too late to save the first one).  He screamed for about 48 hours, the way a starving kitten screams, like on automatic pilot -- I think nature does tht so it will call their mother if possible -- hard to understand how a mama cat could ignore that but they can.  Anyhow, I had to bottle feed him to raise him.  He ended up staying very tiny, no bigger than a 6-month-old kitten, and he had strangely deformed feet -- he had like "mirror toes" tht came out of his pads and came up to meet his normal claws.  They were made up out of some kind of callus material but they made his feet loook like crab claws.  He was a really sweet cat though.  He vanished out of a completely fenced enclosure, and even though he was wearing a collar and a tag, and we had every kid in the neighborhood looking for him so they could claim the $100 reward, he was never found and neither was his collar and tag.  We suspect a large predatory bird took him because we couldn't figure out anything else.  It was shortly after we moved to the country.  We had a fenced enclosure in the city too, but I don't think the city birds were as predatory as the country birds are.  We have eagles and hawks out here and I didn't think of that.  He was so much smaller than a normal cat but he was an adult, over 2 years old, and I shouldn't have let him out with the others but never thought about BIRDS getting him!  We were safe from dogs and coyotes as the fence was 6 feet high and was curved in so the cats could not climb out.