How about putting a paper collar on him, asking the "owner" to call you?

Can you get close enough to pet him? Are the pads of his feet VERY rough (this can be a good indicator of a stray or lost cat that has been wandering for a while).

Is there a local shelter who could help, or do you have a garage where you could keep him away from your cat whilst still helping out - maybe the vet would help with the costs if you explained the situation?

If he does turn out to be healthy and homeless, are you in a position to keep him?