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That's a tough one for sure...
I can only think to give you advice through a story. My freshman year of high school, I earned the spot of a starting running back during my pre-season workouts. I worked hard all summer and leading up to the season all I cared about was football. I then had a check-up about 8 days before the opening game - my lung function had dropped. I was given a "choice" by my docs and by my mom - go in now for a short stay, work my butt off and be ready to go for the second game of the season, or delay the stay, possibly get sicker and end up being forced to go in and miss 2, 3, 4 games or more. I say "choice" because my mom had a good way of getting me to choose what she ultimately wanted most of the time. I think it was a healthy balance of love and fear that I had for her that was the trick
I chose to go into the hospital and I ended up missing the first game of the year. The coach stood behind the decision of of putting my health first and held my starting spot for me upon my return. I worked my tail off in the hospital and ended up making it a very short stay. My lung function shot up and I felt great. The dangling carrot of that second game was out there, and for me, it worked! I went on to play in every other game that year.
Looking back on my regrets from when I was that age, I never once think about that decision. What I do think about are the times that I didn't put my health first. I have the "what-ifs" and "if only I would haves" that will pop up from time to time.
On the continuum of time, the start of a basketball season is but a speck of dust. Admittedly, it looks like a mountain range to your son, and that is completely understandable. I couldn't fault him for either decision that he would make as it's much easier to be a Monday morning quarterback sitting here today
I can tell you this, the people in my life now are VERY thankful for the times that I put my health first. When I compare that first football game of the season to my wife's kiss or my daughter's snuggle, it really puts into perspective how insignificant the game, the sport and that time really was.
At 18, you're not only making decisions that will affect your future, but the future and happiness of others.
So, to finally answer your question...As a son, I always expected my mom to be loving, respectful and honest. Kids are always more open and responsive when their parents show empathy and not "I'm the boss" in this situation. (With that said, may I remind you that you are in fact the boss if he still lives under your roof and you're paying the bills).
Hope that little story helped prepare you for your conversation with him and please let me know how it turns out. If there is any other question I can answer or anything else I can do, please let me know!!