I don't understand... potty training related

OK, so sometimes it works out sometimes it doesn't. The past couple days Paul has been peeing in underwear constantly it feels like. He is just playing then pees. Normally he stops himself after he feels he is wet. But it is frustrating! I really don't want to give up. I feel like he gets the pee thing and he is amazing at pooping in the potty (and I don't want to lose that), but seriously how long will it take to untrain him to just peeing wherever? Cause I think he just forgets that he just can't pee wherever he is (like he could with diapers). When he comes to me saying he has peed or says I'm peeing (when I'm close), I make him take of his wet underwear and then sit on the potty until he pees more. I just made him practice playing and then running to the potty when he needs to pee.

I've also bought him a train and a cake (when you ask him what he wants he says that) and I plan on displaying it (train will come tomorrow) and giving it to him when he has successfully had a day with no accidents or when he can take himself to the potty when he needs to go or something like that. Cause he still hasn't decided he needs to go and taken himself to the potty yet. Seriously, the other kids got the pee thing quickly and struggled with poop and he is the opposite which is actually more of a problem (and more frustrating) cause of the number of times a day you pee! Grrrrrrr!!!!!