Its always a good idea to have them vet checked. If he is scratching himself, then he may have parasites which need treating.
If he has had a fight with his cage mate, then you still need to have the wounds checked in case of infection.
If he is fighting with his cage mate, then you may need to take steps to separate them if their bond has failed.
Ive just seen on your introduction thread that you have three guinea pigs. Are all of these piggies male? If so, then this is why there could be fights occurring. Keeping three boars together almost always fails with fights occurring - boars do much better when kept in pairs. To attempt to keep three boars you need a cage of three square metres but if they dont have character compatibility (which most boar trios dont), then no amount of space is going to make any difference and they will need separating.