Philtrum
have you had him checked to see if it's actually colitis that he's suffering from? A couple of the tell-tale signs:
(i) if there is mucus on/in the poo;
(ii) if there is any fresh bright red blood in any of the poo.
If you settle on Arden Grange, one of its advantages is that it's available from Waitrose too.
I feed a raw diet because Paddy had colitis from ten weeks old, and it's sorted him out. Whichever food you go for, check that it doesn't have artificial additives or preservatives in it. Can make them a bit hyper and not good over time for their bodies.
Keep us posted.
Denise