I'm thinking of blocking my sons father on my sons phone
What reason could you have for blocking your son's father on his phone?
He rarely makes an effort to contact his son. This is totally his fault.
Your son's relationship with his father needs to be worked out between them. You blocking your son's father on your son's phone only interferes with this process. Keep out of this.
He's a deadbeat dad.
That is not your call to make. You shouldn't interfere between father and son.