Was he circumcised as a boy? that is the problem with circumcision that a lot of people dont know about - a large percentage of the nerve endings in the shaft of the penis are in the foreskin, which is removed by circumcision - I hope for both of your sake he still enjoys having sex with you - does this lack of feeling interfere with your enjoyment? That would be a bummer - to not be able to feel anything on the shaft! Check with a neurologist - they may be able to suggest something!
Be careful uncircumcised fathers!! My son was circumcised for vague medical reasons at age 6.
Unknown to me at the time (many years ago) it caused me major psychological problems and almost made me impotent for years just thinking about his foreskin being cut off.
Now he wants to circumcise his son my grandson and it's all starting over again.
My advice to fathers, if its not in your culture or religion don't do it.