Sit him down on potty or, if out and about, I would still get them to crouch down as they often don't wee much and until they have a strong stream and can learn to hold their penises they will tend to wee on their clothes. On the other hand, two out of my three boys were born with a deformity to their penises which meant/means their wee doesn't come out straight so maybe I am facing more of a hurdle than other people.
Just posted this on another thread re whether you first teach your son to wee sitting down:
Well I did but my ds1 had a deformity of his penis and couldn't wee straight. Ds2 just liked sitting down and now ds3 has the same deformity as ds1 but his wee comes out even more crookedly and he often wees over himself even sitting! It is a pain in the a**e.