Remember it is not always about the reputation nationally of the league It is about playing time. No point in playing in the Cape if you never see the field.There are lots of local leagues were D3 players play,Were we live there is a local league, OICBL, that is happy to take D3 players. I checked your profile and saw you are from Houston I did a quick search and found this league, If his coach does not find a team for him, you may consider this:
http://houston5abaseball.yuku....-LEAGUE#.UWYXRpNvPa0
http://coastalbaseball.com/
Do some research and and be sure it is the right fit.
My son is a sophomore at a D3. His coach gave him a lists of teams, during the fall of his Freshman year, and he did not like the options. He talked to his 18u coach from his rising freshman summer. It turned out most of the tournaments they playing in used legion rules. So he played a second year of 18u travel. It did not hurt him for this summer. He has signed a contact with a Great Lakes League team.
Enjoy his time and get to as many games as you can.
Good luck.
Take a look in the after high school forum, there is a lot of information on Summer teams in there.