How do I deal with a dog with nervous aggression???

I've taken in a cross collie / german shepherd 7 month old dog that needed re-homing,,,, only got him last week,,,, he had lots of probs - very nervous, seperation anxiety, toilet training which we are working on and seem to be improving. The problem is he is very aggresive towards youth males (I ume something has happened to him with this age group / gender) problem is I have a 16yr old son and he growls at him, he eventually comes round but next day after he hasn't seen him for a while, he goes back to being aggressive - he's like that wih all male youths - but he's brilliant with kids and females, wee bit wary of adult males with exception of my boyfriend... Any ideas how I can deal with this and give the wee dog a chance?????