build an outdoor pen. It will keep other cats away and him out of the house and safe. If you go to wal mart or home depot you can buy the wire mesh cheap for it, just get 4 stakes, put them in a square or rectangle or whatever and staple the mesh to it. Use an old sheet or cardboard for the roof to keep him in, but something detachable so you can take him out when you need to. This is the same thing ive seen people do for other small animals like rabbits and pigeons. It works pretty good. When he is old enough to get fixed, then you can bring him in.