You have had a neuro opinion which has ruled out any internal problem, so theres not much to worry. The sensation in the hand does get reduced sometimes due to various causes. I can suggest a few exercises he can try:
1. Roll wooden rods of different diameters with the hand on the table.
2. Rub sand paper on the hand.
3. Take some dry sand in a bowl and ask him to take sand in a fist and try to hold as tight as possible.
4. He can try exercise 3 with different kind of lentils and dals.