T.S.
The thing about standing to pee is it makes peeing and pooping TOTALLY different. He has to go in with a plan and you can't just let whatever happens happen.
I'd train him sitting so he can just relax and do what he does. ONCE he's trained and knows his body signals well enough to know when he's gonna poop and when it's just pee, THEN let him try standing.
Also, you don't want to have him stand at a potty chair, so standing to pee is only for when he's tall enough to stand in front of the big potty.
HTH
T.