My son has had a lump on the back of his head since he was born he is now 8 months old is it normal?

Lymph node. If it is no larger than a pea and is mobile, it is a lymph node, (especially if it is on the back of the head) this is a common concern, don't worry unless it gets bigger.
Probably. It is either an inclusion cyst or a lymph node. If it is not red, painful nor growing, then it is probably ok.