How do I get my dog to stop going to the bathroom in the house?

He is a 15 week old lab/pit mix. We have had him since 5 weeks and I don't know what I'm doing wrong! Example: I woke up this morning, went right outside with him and he peed. Come inside, feed him, he eats only a little, wait a few min, take him back outside and he pees again. I open the doggy door so I can run an errand and I come home and he pees on the wood floor, and poops three times on the carpet! When he sees me he runs from me and cowers cause he knows it's wrong! I put him right outside and say potty a million times! I think he does it out of spite! Help!