Potty Trained?

Benjamin has been fully potty trained for a couple weeks now ONLY if we are at home and he has no pants on. The second that we put underwear on him he has accidents (he will go sit on the potty with his underwear on) and he has never gone on the toilet. Sometimes he will go to the toilet and lift it up like he needs to go and then run away saying "no no no, nnnooooo" afterwards he will just use his potty. I know that he gets it but sometimes he is just like me and needs to know certain that he can do it. So, my question here is do we now teach him to stand so he will maybe start using the toilet or should we get him potty trained with clothes on? Never thought that I would have to ponder this...