To become an NHL player, Milano needs to work on his strength and he needs to improve his play in his own zone and without the puck.
I don't know much about Plymouth, but the problem with the CHL can be that players who are very skilled offensively can play a huge number of minutes with 2-3 games per week and lots of travel. That means they can easily float and cherry pick against the weaker teams and pick up bad habits. A lot of the top teams can play a run & gun style with an overpowering offensive force. Coaches want to win and aren't going to press their offensive stars to play in their own zone with such a demanding schedule. They have grinders to do that. A player like Milano could end up with huge offensive numbers, but not developing as a player and no time to work out and build strength due to the grueling load CHL players have to deal with.
The OHL works more as a development route when players are there for 3-4 years, not stepping in as the star likely after the season has begun.
The major plus for Milano to go to Plymouth is he would be able to take part in the Traverse City tournament and training camp, perhaps get in a few preseason games before heading to the OHL.
Playing in at BC seems to immediately address his two major weaknesses. The problem could be is if he struggles and ends up not getting that much playing time for his freshman year.