Motorcycle Safety & Driveability: new bike, dirtbike, new bike

Question
what is the proper size dirtbike for my eight year old son it will be his first bike

Answer
It really depends on his size and overall capability (you're the judge here).  If it were me, I'd size him up to some bikes at various shops and see what I could get that will suit his needs as a learner, but also one that he won't outgrow in one season.