Karen,
He is neutered.
I haven't done any formal training. I rescued him from an awful puppy mill and have basically just loved him lots but no training.
He goes outside a lot -- my husband works from home and so is able to let the dogs out all day long.
When it happens I generally do a lot of yelling and "Bad Dog!" and a spanking.
And it seems to happen in the evening.

I'm really at a loss.