I think you can give a half or quarter aspirin but I wouldnt give anything until you see a vet. Just rest him for tonight. Might just be a pulled muscle and he'll probably be fine by morning.
We were advised by a vet from years ago to give our old girl 1/2 a parecetomol (sp?) if she was under the weather, but when we mentioned this to our vet of 14yrs he said that this can effect their liver, so he gave me a bottle of metacalm to use instead.
Melacalm is very handy to have in their cupboard as when Henry was limping a few months ago I put 1ml on a digestive and the following morning he was back to his old self.
Sounds like he may have strained his shoulder if he's sensitive to touch it, so best get it checked out.
Hope he's ok
