Ideally he will play in front of relevant coaches more than once, if possible. Is he more interested in a local D3 school, or in a broader search (as in HA)? If local, have him do a local PBR (or equivalent) showcase in May, they often get lots of local D3 coaches. Then you can stay in touch with them through the summer - but I agree that those coaches are going to be at 17U events, not 16U. If HA, then yes, Headfirst and Showball. Many D3s have camps in the summer through August, as well as in the fall - you might prioritize those over 16U tournaments, if there are particular schools he likes.
Ask the organization where the 16U team is going to play - some tournaments (especially more local ones) are joint 16U-17U.
How is your high-level organization with D3 recruiting? Some are not that good, in which case you'll have to do most of the work yourselves. Have you asked them what they think about your son? It's a hard conversation to have, and they may want him (or your money) on the 16U team, but if they are being honest, they should tell you whether they'll be able to do for him what you need on the 16U team.