My 5 year old son had to have boxer briefs just like mine.  I've tried other brands but always come back to Hanes for a nice fit and comfort.  My son has no complaints and they fit him nicely too.

Update 29AUG13:
 I bought my son another pack of these in the small size since the extra small's seems to be getting tight on him.  The crotch and waist on the small's seem to be a bit big (wish there was a size between) but they still fit him and he as room to grow.  I have had no quality problems with any of the initial pack I purchased last year.  Also, do not expect to get the color pictured on Amazon as Hanes seems to package colors at random in their 3 to 5 pack sets.