Help/opinions

Ok so I have a 3 year old son who's father I am no longer with. My ex is a great dad and tries his best however has some serious temper issues.

Today, I sent my now boyfriend to pick my son up from his nannas house as my best friend was in hospital. My son's dad still lives with his mum and happened to be there when my partner turned up.

Cut to the point, apparently my son's dad didn't like the fact my partner was picking my son up and dicided to physically assult him. My partner been more mature walked away and didn't react.

Ok, so now the police have been called but how do I respond in regards to contact between my son and his father. Punching someone is no way acceptable and I'm honestly worried about how his anger will affect my son.

Opinions?