Before your son accepts an unsolicited invitation, he should make sure that he speaks with one of his college coaches. More often than not, college coaches have well-established networks in place with summer programs they know and think well of.
Many times, the college coach will assist the player in lining up a roster spot with a team that the coach believes is well-suited to the player's development stage. He may also be a good source of information about one or both of the teams that have sent contracts to your son.
Best of luck to him!