Do you have any advice on how to go about trying to regain custody of my 7yr old son please

Who current has custody of him? Is it his father, relative, or the city?

His father

Why does the father have custody?

Social services got involved and I was having problems with my partner at the time (WHOM I AM NO LONGER WITH) then I had to be assessed by a psychologist who said my son was better off with his dad

I would suggest getting an attorney to represent you. Without knowing the terms of the change in cutody its hard to advise. At the least you will need to prove to the court that the conditions that prompted the change no longer exists.