3 year old Pees on himself

Okay so my 3 year old is potty trained and doesn’t put on diaper at home ,only when he sleeps. But for some reason when we go out, he holds in pee and let’s me know last second that he wants to go,and of course it’s too late to get to bathroom in time. He pees on himself in public too many times. I have tried to tell him to let me know, And I make sure he pees before we leave the house .I don’t want ppl to bully him with that, Is it normal things that happens ( 1st time mom) . How do I get him to tell me when he wants to go?