Can you confirm that the mortgage is in your name and the title deeds are in his name, as this is what you are implying.
This would be a very unusual case if it is correct - normally it is impossible to have a mortgage if your name is not on the title deeds.
If the fact is that the mortgage and title deeds are both in his name only, you will need the permission of the mortgage company to put your name on the title deeds. If you are a bad credit risk they will refuse.