A player has a spot on the roster when he’s handed a uniform. While rare, scholarship players sometimes don’t make the team.
You don’t have an offer until an offer is made. What your son has is quality interest. The coach could become more interested in someone else before he commits. The coach said, “When I offer a roster spot ....”
No money, but will “most likely” have money next year means nothing until money is on the table.
D2’s aren’t funded as well as D1’s. I would do some investigating regarding how many players typically show up for fall ball (over recruiting).