I'm going to block my sons father on my sons phone
Why are you going to block your son's father?
My son's father is accusing me that I am telling my son not to communicate with him.
Blocking the father of your son means they will not be able to communicate anymore. Before you do that you should explain it thoroughly to your son.
I don't think I need to tell him anything. The father is a loser anyway.
It's hurtful and damaging to your son to block his communication with his father. You should try to handle the situation with maturity for your son's sake.