I will not comment as to if is a money grab or not. The USHL has made efforts to insure that promising 15 YO's stay with their affiliated AAA midget team or local high school programs. Of course their are exemptions, but your son is a bit young to be at that level.
If good enough, he will be contacted by an individual, scouting for one of the teams, or at a later age, a college scout looking to have him mature for a season there.
There are many questions to be asked of this try out camp...location, duration, amount of prospectives, total amount of icetime available etc.
To a young talent, this invitation may seem flattering, but it is you and his local coaches that need to keep him grounded. Don't be afraid to contact and research as much as possible.
s...