Slacking in the potty department

My three year old son has been potty trained now for exactly two months. He started off great, always staying dry and rushing to the bathroom when he had to go. Now he has started slacking off and will pee a bit in his underwear before getting to the bathroom. It's not enough to come through his pants, but it's still enough that he should feel uncomfortable.

If I don't notice, he will keep the wet underwear on and it doesn't seem to bother him. The funny thing is, he only does this at home and he rarely wets the bed during the night. When we are in a store or the park, he tells us in plenty of time to get to the bathroom. Yesterday, when I talked to him about why he does it, he says, "Don't worry Mom, the pee keeps me warm." I couldn't help flipping out about that one. So my husband jumped in and talked to him about going to buy baby diapers since he couldn't stay dry and my son got really upset and said it wouldn't happen again.

This morning, he peed on the couch and didn't tell anyone. Now what???