Thank you @MServant , I had hoped to hear that the gentle squeezing had been done and hear comments from the OP, whether squeezing the sides of his ribcage causes the center to protrude.
@SuperSam Has your vet taken an Xray of Ace's chest? I have read a number of other threads where people have followed catwoman707's advice and it has made an amazing and rapid difference to so many kittens' life. I do have a little anxiety for Ace however - in case his ribs are not only flattened but also have any curve inward which might make the use of a little jacket inappropriate. If an Xray hasn't been done please do ask your vet whether they think Ace has 'Flat Chest Syndrome', or 'Pextus Excavatum'. However, making one using catwoman707's instructions and fitting lightly to prevent Ace's body weight pressing down on his ribs and aggravating the problems he is having for a short period until you can check this with the vet should not do anything other than help your little guy feel easier in his breathing.
I was afraid to mention any other at this point so as not to discourage helping this kitten since there are clearly some hesitations now.
The "deformity" has not been elaborated on yet though....waiting/hoping.