a. There's no guarantee of anything even after you sign a contract. At that highest level, your player can be cut at any time even after the season has started. You aren't really safe until a roster freeze period. Even then there's usually 0 guaranteed you'll be played (some 2nd and 3rd GKs for example don't see time on the field during regular games).
b. If you concern is starter v. bencher MLS Next Teams (especially once they pull past U13) carry far more number of players than are able to even dress for a game. At my son's old club, for his age they carried 29 including primary, future, guest and reserve rosters (reserves only brought in if someone is on injury list).
c. Primary players are hard capped in numbers, can play all MLS Next games and cannot play for another team down
d. Future players are capped at the number of games they can play (I think 12) in MLS Next but they can go to MLS Next Tournaments and can be borrowed down in certain instances (it's why second teams of MLS Next Clubs sometimes perform irregularly....it depends who they have on any particular day).
e. If he remains a future player instead of being brought up to primary, depending on his age either you or he will have to have a discussion with the coach about where he'll get the play time (unless he's happy riding the bench full time with just a few minutes here and there)l