Depends on the time of day and his level of activity. I think the accumulation of urinary events over a day will lead to a musty odour, but this should be mild.

Does he have conditions such as jock itch? That can cause smells. It's basically like toe jam, but in the crotch region.

Also, if he has opened his bowels at some stage he might be emanating smells from that area too.

Good luck