Big school, small school and in-between, development is on the player.
Find good personal trainers (budget allowing), physical and baseball.
Sit on his academics - they will be as important to his chances as baseball. Meet with guidance counselor as early as you can get an appointment so the family can get to understand the college selection process (most rigorous cirriculum, rigorous, etc. levels and their meanings). Also, take in a few local college games so you'll can get a feel of the level of play needed and visit lots of various college campuses to get feels for different types.
Dont worry about size. Mine (LHP) was 5"3ish and 125 lbs heading into HS; some of his earlier blooming friends were seemingly twice his size. You cant control that; you can control grades and hard daily baseball related work.
Also, welcome to the site.