A last update, took him to a specialist vet who discovered a lump in his mouth, very likely to be a tumor. He's been home this weekend and the lump has grown bigger very quickly (hard lump, no redness or real pain reaction when touched so I think it's most likely to be a tumor). We will be saying final goodbyes to him tomorrow.
I'm gutted, I'm so upset about it but I do feel that it's the right decision to make. Going to miss the little hammy so much