There is no harm in trying the solicitors. They will always point you in the right direction. The executor may have their own interest in the contents of the will, so I'm not sure how hard they have to try to find anyone else who has a stake.
If he doesn't want to deal with his father, then his solicitor will deal with it. If he knows he has a stake in the will, then his solicitor should be able to ensure he gets his bit. something to do with probate applications... I think...