No, you cannot just bring him inside. First, he may be someone's outdoor cat. And if you cannot pick him up, he may be feral. You are risking disease if you bring him indoors without getting him fully vetted. What if he is FeLV or FIV positive? He could be carrying the upper respiratory diseases, fleas, ticks, and parasites. He would need to go to a vet before you even considered bringing him indoors. Your best bet is to either leave him where he is, and provide food and a shelter for him (great ideas on may web sites with making a shelter out of a Rubbermaid container). You can trap him - shelters will loan you a trap - and take him to a shelter, with the instructions that you want him back if no one claims him. You would need to get him neutered if he is not, vaccinated, tested for FeLV and FIV, dewormed and flea treated - and microchipped. They will also be able to tell you if he is likely feral or not. There is much more to this than bringing him indoors as you put your own cats at risk. Many places provide low cost spay/neuter, and some shelters with vets on staff provide low cost basic services, but other than spay/neuter you are generally on your own when it comes to vet cost.