Darn ... clicked wrong button - didn't mean to "like" this, tried to reply.
Anyway, if mom and dad report him as a runaway, and the law permits MA cops to collect runaway juveniles, they will - and, legally, they may be obligated to. They would turn the child over to child services, and then child services would make arrangements for repatriation or other arrangements as the parents and the state of MO might choose.
If we got a runaway minor from MO here, we would be obligated to take him into custody and turn him over to child services. They would also have to either return him home or place him into a group home or foster care until arrangements for his return could be made. In my state we are unable to release an unsupported minor child onto the streets.
But, as has been pointed out, if mom and dad are supportive of his actions, then there will be no runaway report. No missing person report means no initial police involvement. And the issue is entirely moot unless he gets into legal trouble or needs medical care in MA.