I block off a hallway for my ratties to freerange. I block it off with two thick pieces of foam board. It's a ratty range room when they are playing and everything is easily put away after. Any way you could do that? YOu can also do that with a corner of a room. If you are worried about the carpet, put a blanket down as well. You can also let him run around on a bed, or use the whole bathroom, or a walk in closet. The possibilities are endless if you keep your eyes open. Make sure to put lots of toys around where ever it is. I buy baby toys from the dollar store. I also use large industrial cardboard tubes I get from work and paper bags I get from shopping or work.
Hrm, rats aren't that expensive so I can't understand why you can't afford to buy him a friend, unless it's a cage space issue or another cage that you need to get.