Train him to sit down when going to the toilet, always. Makes him go to the a public women’s toilet for every time he doesn’t. If you catch him not sitting down, then makes him wear Diapers.
If you are disgusted by changing him, make him change his own diapers once he’s used them and gives him a time limit. Use the diaper treatment every time he shows he can’t go to the toilet the right (like a girl) way, or whenever you find any drops, spots, residual marks in and around the toilet after he uses it.
Train him properly from the beginning, and only offers to give him another chance to be trained again after completing 24 hours in diapers. He will not forget next time, that's for sure…. And your toilet will remain spotless!