I agree that it probably would not be in your best interest to contact his parents.

All you can do is express your thoughts / feelings on the matter and then let it drop. Have you explained to your BF that you are concerned, and it affects you because you care and you are a team. Your BF is a grown up and has to take care of himself.

If it hurts you too much to see / hear what is happening, then (unfortunately) you'll have to distance yourself in such a way so you can manage things.

I wish the best for you and your bf