[Hammy the great]

Hi there
Firstly want to say what a great forum, very informative...
So I am the owner of Hammy, along with my 11 year old son.
We have had him a few months now and already have upsized his cage and realised that the cages that PAH sell are far too small !
So Hammy is a Syrian Teddy Bear and is quite shy around us both, he sniffs plenty and takes food and sits watching my son on his computer so is not scared but just isn't up for being handled too much.
So yesterday when he was having a little climb I noticed that his bum area was dark and on closer inspection looks like he may be a little constipated. I have bought a small pet carrier so I can clean with some warm water and get a better look at what it is. Last night he was running round as normal and in his wheel and he has been out this morning for his normal school morning stroke and goodbye. I have noticed that he hasn't eaten his nuggets food that I changed to ( he was seemingly fussy with the mix) but has eaten the food that I hide in his cage and the small bits of veg that I put in. I put some of the mix in this morning and he has eaten this. So my question is at what stage would I be looking at a vet visit?
He is fine in himself, the water bottle is working fine, the only thing that has changed is the food.
Any advice will be great thanks