Difficulty with potty training 3 y.o

Hi dads, My boy is 3.5 y.o and we're having some difficulty with potty training. He's pretty good at telling us when he needs to pee and we usually get on the toilet on time, but where we're having some difficulty is that he REFUSES to hold his own weiner down. He NEEDS me or my wife to push it down for him. We need to train him to hold his own penis (for obvious reasons -- preschool, babysitters, sunday school, growing up, etc.) but he aggressively refuses any time we try to get him to do it on his own, and it ends up turning into a big mess in the bathroom. Has anyone else had this problem with potty training toddlers, and any advice? We've been potty training for about a year now.