What breed dog do you have?Was the dog shaved and now the hair isn't coming back in, or is he losing hair in one spot?
How old is he?
Thanks for answering my questions.
Hair loss like this can be caused by a couple different things:A thyroid problem...which is pretty doubtful because your dog is so young.
Trauma to that skin...which means that perhaps when he was being groomed 3 months ago, there may have been trauma (razor burn or too hot of a shaver) that may have damaged the hair follicles in that area, making it so they don't grow back.
Possible mange...which is a possibility if the area has gotten bigger at all.
Because we're no exactly sure what could be causing this hair loss, the absolute best thing to do is to have your regular vet take a peek at him. It could be that he may not grow hair there again...but it's always better to have the dog looked at...just in case.