Oh no, another potty training post

Many setbacks with potty training due to illnesses, etc. So oliver is fully trained wearing boxers (boxers work better with him than underpants) .... although he takes them fully off when he goes instead of just pulling them down. Could be because our toilets ate slightly too high even with a step. He likes to climb up himself. (We scrapped the potty as he's so tall) I don't mind, though, at least he is going. He tells me he needs the toilet and off he goes. I've tried loads of times to move on to next step of wearing trousers too but he just wets straight through them. (I've even tried trousers with nothing underneath ) He won't even attempt to go to toilet or tell me he needs it, just wets them straight away, and it doesn't even bother him either. He won't even try to remove them. Help!!!