I am glad you are aware that high myopia in a child needs special observation.
When you talk with the doctor in the next month, you may want to talk with him/her about what is causing your son's myopia (either the length of his eyeball (being longer than normal) or the natural lens inside his eye not focusing the light in the right location on his retina, or another reason). The eyeball being longer from the front surface to the back where the retina is located is the most common cause of myopia.
If his eye is very long, you should discuss with the doctor the risk of his being hit in the head during a soccer game (by the ball, by being tripped or falling, etc). In some myopic patients who have 'long' eyes, hard blows to the head can increase the risk of a detached retina.
Detached retina is a rare in children so it's probably nothing to worry about. I would just clear with your son's doctor the specific sports he wishes to participate in. The sports glasses are a great idea to protect his eyes generally. You should just ask if there are any other precautions to take. Best of luck to you and your son.
I will ask him next month when we have the next appointment
Has the doctor agreed to him playing with the sports glasses? I guess it's fine as long as the glass wouldn't break easily.