The better they get with training the longer they are able to hold it. As he gets better and better, you'll totally be able to take longer walks and go out and about. DD trained last year and at first it was RUN to the nearest potty as soon as she said she had to go. Now, if we're out and about, I take her to the bathroom when I go, and she goes, and that's it. No real issues. Sometimes when we're out food shopping she says she has to go, and i take her.
But in the meanwhile, it's GOOD that he recognizes he has to go. It might mean a few accidents when you're out (cloth trainers or pullups are a godsend for times like this) b/c they can tell you they need to go but god forbid they have an accident it's easily managed. We did pullups for day trips well into potty training. As long as you treat it like underwear, they'll treat it like underwear and they can be a useful tool.