Potty Training

Okay so Jax is now 3 years old. I have been trying off and on the past year to get him to go potty in the toilet. He had a little while where he was doing really well. He wanted to go and would tell me. Then, it stopped. I ask him every day if he would like to go potty in the toilet and he just tells me no. I bought him a baby toilet..he is too big for it. His fat little butt is too big. Me and Taylor thought that a little toilet woule be the key but oh no. I don't know what to do. Saturday afternoon, I didn't put a diaper back on him and thought that he would feel a little tinkle and would maybe want to go. Oh no....he just peed in his shorts and was hanging out in them. It was gross. Does anyone have any ideas to potty training? I know a lot of people say to just let them do it on their own time. At this rate he will just pee his pants until he is old. He will never have to get out of a diaper. He will just change to the old people diapers when it is time. Gosh.