This is perfectly normal at first. He probably can hold longer, but you have to remember he's never had to, so doesn't know how to. He will learn how long he can wait in time, slowly extend when you ask him if you like.
PT 2.8 year old for 3rd attempt, going great but after a large drink he needs to go after 20 mins - I'm confused, as advice says he should be able to hold it for at least hour and a half. He can hold it in for a lot longer if he doesn't have much (literally a few gulps). We're only on day 4, everything else is great otherwise!