I think it's ok to call the doc. They can make the best decisions when they have the most info. Obviously he can't talk to you about your sons problems without his permission but you can inform him of what's been happening so he knows what's going on when he sees him. Do you think your son won't be honest with him? Also do you think your son would be angry at you for ringing?
Ultimately I might be a bit annoyed if someone rung my pdoc about me but I think I'd see it as them caring about me and trying to help me.