The day after my last post (sorry for the late reply by the way!), I checked his foot and the swelling had gone down a lot and the colour was a lot better too and now it seems to be fine
So I didn't take him back to the vet but I'll keep an eye on it just in case. I'm using Carefresh at the moment so none of the fluffy bedding.
Luckily I did have a spare wheel, it's plastic, seems pretty sturdy and there is nowhere he can get his leg trapped or anything so I am using that until I get one of the Silent Runner wheels
One more thing about his feet though... his nails! He has a sand bath and some rocks but they don't seem to be keeping his nails from getting too long so I have to trim them. I just do a few nails at a time so I don't have to hold him too long and that's working out okay but I think it would be more beneficial to him if I didn't have to do that. I've read that giving a hamster a shelled walnut is a good way of wearing down their nails. So would it be okay to try with my Roborovski or can walnuts only be given as treats to Syrian hamsters?