I would take him back to the vets, and if you aren't happy with the diagnosis, ask for a second opinion or referral. Was the diagnosis made by an xray? When you say enlarged colon, do you mean it was inflamed, thickened, or dilated? There is a condition known as MegaColon.
I also disagree with the diet advice you have been given, further thinking you need another vets opinion. How long have you been advised to keep him on chicken? Chicken meat alone is in no way nutritionally complete, balanced or nutritionally adequate for any cat, let alone one so young.
Do let us know the outcome.