I would urge your friend to keep a diary of what’s happening. Not least because one day his son may need to know he tried
Five years ago, my best friend became the father of a baby boy with his then partner of over 10 years. When the baby was born, his then partner registered the birth and, without telling my friend, omitted his name from the birth certificate. The relationship broke down and he left the home they jointly owned.
They remained close, although there were some indications that the relationship was dysfunctional: he hinted at financially supporting her as well as the baby (despite her having a full-time job) in exchange for contact with his son, and limited contact with the rest of his family.