I used to think he was a genuine idiot and then came to believe that his humour revolves around playing the fool, a sort of court jester, just to enjoy the reactions.
The only problems come when he starts to discuss something seriously when he manages a 60:40 split on fairly sensible:as daft as a brush like you always thought he was.
I would buy him several pints but probably switch them to alcohol free towards the end of the evening.