Potty training. Position during potty.

My son is 2.9 years old. I have started potty training. But so frustrating he is doing pee in his small potty when he is completely butt naked. But when I put him a underwear then he wet that . He doesn't understand that he needs to tell me that he go to toilet . So many times I said him that when you have need to go tell me . And he is not doing potty in his small potty . When he is butt naked he ignored his bowel movement so he suffered from constipation. I give home lactulose. But still he didn't do potty in his small potty. Either he doing in his underwear or doing in his diaper. He doesn't want to seat on the potty . He is doing his potty in his standing position . So how can I make him sitting position during potty.