I would be willing to bet he is not neutered...... I knew a springer spaniel who was the same way till he was "snipped", he even tried to hump my son while he was asleep (who was 7 at the time) when I was dog-sitting him. Neutering him would probably (more than likely) curb it.

This behavior is often a sign of dominance.  It is never too late for obedience training.  This will establish YOU as the alpha dog.  Once he understands that you are in charge, he will understand his place in your pack/family and the behavior will stop.  I have rescued pits, and my female will hump my male dog b/c she is the more dominant of the two.  She will also stop if I correct her b/c I am the leader of the pack.  Don't get me wrong, they are my babies and I love them to death, but dogs are happier and more secure when they understand the social structure of their home.  Remember all dogs are basically descended from wolves which run in structured packs, too.  Good luck!  I LOVE Am. Bullies!!!

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Joy