Sounds like a leak if it is only around the hip area, KWIM? If bubs was sweaty, surely there'd be more than one sweaty patch?
Could you try extra boosting? (Sorry, I am generally ignorant of 'boy' issues, LOL! But...) Are you 'dressing him down' (i.e. so he doesn't pee right out the top)? Do you have other types of nappies you could try to see if there's a better fitting one? GL with it!