My son just turned 8 ten days ago and doctors said he has started puberty

My son just turned eight a few days ago. He was diagnosed with Asperger's two and a half years ago and when he went in for his yearly physical they said he had already started showing signs of puberty.  His facial hair (he has had a small mustache from birth) is darkening and he has curly dark hair around his scrotum and under his arms.  In the last 4 months he has put on 10 pounds, which is a lot for him, he is scrawny built.  And he is standing at 4 ft 11.  He is having trouble controlling his temper already when he has always had great control of. What can I expect in the next few years? And how long does the change take?