[MobbDeep]

Bet $20 on flop and after that it's much easier to put him on a hand depending on his actions.
There is no way you can know what he is holding now. He might think your small bet on flop indicates weakness or he might aswell have a very good hand you just can't know.
If you bet $20 and he raises you have to give him credit for having a good hand, in that situation I would probobly call and see what happens on turn...

But the thing is that he would probobly not try to steal this pot if you would have bet $20 on the flop.