As a pup he his food should have a higher ratio of protein than adult food, combine this with the fact that he is probably still crouching and the urine will burn more than when he is adult. The only thing I found was to water the areas he uses to disperse and dilute the urine, rake out burnt/dead grass and reseed areas that were beyond help.
It will get better when you switch to adult food and he starts to cock his leg. But until then......!