I don’t think it’s going to hurt his long term development by giving him his nine, or even letting him get his teeth wet playing 9:00 for a full season.
Being around pros is a good thing for him because it lets him know how much he needs to put in to grow as a player. It also shows him the lifestyle and gives him something to work towards.
Throwing him into the deep end unprepared with full minutes no matter what, also known as the Kotkaniemi, will hurt his development because his bad habits will be excused and allowed to take hold, and then next year when there’s a full year of tape on him he hits a wall.