Runny Poo in 2.5yr old-poss food allergy?

[usuallyalurker]

DS2 aged 2.5 was diagnosed with a peanut allergy 3 months ago and has asthma, excema etc. He is still in nappies and most days has very soft or runny poo, sometimes he does a hard formed poo but more often than not is is very very runny, still like baby poo. He is fine, happy and generally healthy but i'm starting to wonder about the poo. He eats an OK diet for a 2 yr old, with fruit, cereals, limited veg, meat etc but he does have lots of dairy. He likes a lot of milk at bed time and in the morning and eats yogurts, etc.
I'm wondering if this runny poo is normal, or could it be caused by his diet. Could he have a food allergy? Milk intolerant? Does anyone have any experience?
Not been to the Dr yet, but we are kind of starting potty training but I don't know if I can fact pooey pants until it is a solid poo!
Any experience or advice gratefully received.