Regardless of HS Summer Ball or Travel Ball, development has to come from the player 1st and foremost thru effort and commitment that seldom have that many wish they had.
For him to fully develop over the Summer/Fall/Winter into Spring, he'll have to work hard when neither HS or Travel ball are going on b/c there isn't enough time regardless of either direction to fully develop.
Whether that means a players needs improve his arm strength and throw long-toss off a brick wall or him and a buddy go a local cage and work on hitting by doing tee/soft-toss drills etc.
True maxmimum effort will go further than deciding one way or the other between Travel ball or HS.