My 28 month old has always refused to sit on the potty so for the past couple of months we would stand him to pee in his Bjorn potty right before bath time. I have to hold his hips to get his aim right.

We tried getting him to sit several times but he fights us and holds his pee until he is standing. We havent tried poop because he always stands to poop in his diaper (he bears down pretty hard).

I want to start serious potty training, but he refuses to sit to pee OR poop. I am worried that he will make such a mess when he finally gets to the point of peeing by himself. Also he is about to start preschool, where they will help train him, and I dont know if they can help him aim every time.

What should i do? Just continue with the standing to pee or try to get him to sit to pee?

Just to add, his pee doesn't come out straight, kind of at an angle, which may make it harder for him to learn to aim on his own. I have to hold his hips to prevent pee from going down the side of the potty.