I'm having a lot of trouble potty training my 3 1/2 year old son. He doesn't tell me when he has to potty, which doesn't help. I've tried pull-ups, putting him in briefs, having run around the house naked, and having him sit on the potty every 30min for about 15min. Nothing I've tried works, and it doesn't bother him at all to wear a dirty diaper all day long. What should I do?