My son is refusing to see his father after he found explicate text messages to fathers girlfriend and thinks her children are more important to his father than he is?

Can't happen unless there's some step-relationship here, such as a man marrying his step-mother, the wife of his father. That still makes you your father's son in any case, but now you have become...

He would be a grandson.

see link below

of course

there probably planning a gift or maybe your girlfriend is into older men