Throw a Cheerio in the potty and tell him to pee on it! My son thought that was funny.

Try letting him sit backwards on the toilet..then try to transition to standing.

I agree with Emily above, try to turn it into a game. I had my son shoot at a square of toilet paper (which also helps to reduce splattering) and he enjoyed that because the toilet paper would fall apart as he "shot" it.

The other side isn't necessarily better. My son stands but doesn't direct it so urine ends up everywhere! Sitting might be a good thing for now.

Why not let him continue to sit? It's a lot less messy LOL