Others may have had different experiences but from what I've seen and heard, there is a firewall between D3 coaches and the money, be it need-based or merit-based. In other words, the coach has no standing to make what you're calling a "proper offer", other than saying that he's got a roster spot and possibly a guarantee that he'll be accepted, which can be invaluable at schools with <10% admission rates, even for fine students like your son. In many if not most cases, colleges have online tools that can give you a pretty good idea of what kind of aid of both types your son may qualify for, and if you need more specifics the coach would give you the correct person to contact about getting a pre-read so you can have more concrete numbers to consider.