What happens if you leave him for 15 minutes? Has he ever fallen back to sleep at this point? Does he have a paci? Is he swaddled? Could you try white noise in his room? Is there anything that's waking him up and stimulating him, like noises or even toys in his crib? Can you extend his nap using pat/shh?
Sorry for all the questions, just throwing ideas out there. The other thing to keep in mind is that babies brains develop the day sleep part between 3 and 6 months, so this might just be a developmental thing. Don't stress too much about it, just keep his A time shorter following a short nap, and roll with it. Easier said than done, but honestly this will pass.