Let him see different sizes/colours/shapes etc until he's completely happy. They last thing you want is a stag leap over a jump he hasn't seen before just because you think he'll be fine. This will make him lose his confidence. He needs his confidence building, lunge work will help with introducing him to alot of different things.
If he's 100% comfortable on the lunge then get on him and do little cross pole.
If he's 100% on the flat you should be able to make him wait for the stride instead of letting him do his own thing. He needs to understand half halts etc.
Be cautious, grab some mane to stop yourself jabbing him in the mouth.