My son's UK family visa has been refused. They ask for the father's consent.
Other then giving the father's consent, what other requirements do you think they will need in support of my son's application to make it a success?
It is impossible to say which other things you may have forgotten or overlooked -- unless we just replicate the full list from the official guidance, which you don't need to ask here for.
Generally they don't bother to look further at the application once they find a reason that means it has to be refused. So you cannot see from the refusal letter either if there were other things missing or not.