Skiidog,
I got nothing to go on here. When that happens to me, the only thing I trust is my gut. I try to think it through, but since it's untraveled territory, that's all hypothesis and not really very clear...so here's what I fall back on...how am I going to feel about my actions if???
In this case, the if would be, if I withheld this information from a coach. The answer for me, would be that it wouldn't be wise, I should be forthcoming...of course your question is when...well, I think the answer is, the moment it becomes relevant to the coach. It's obviously not relevant if he pats Junior on the head and says, "we'll be in touch, kid". However, the moment he tells you that he is recruiting your son, then it's relevant.
I don't have to tell you it's a tough spot, but I think if you look at it in terms of, "personal conduct", things become much clearer. It's a good lesson for everyone, if they get to see things done, how they should be done. The learning never ends, does it?
Best of luck to you and your son.