Make sure he has plenty of straw bedding in his sleeping compartment. Ensure that the hutch is in a sheltered position away from cold winds and insulate it if necessary.
Put plastic sheeting over the wire side of the hutch so that he can still see out but is protected from the cold.
You can fix sheet polystyrene around the outside (not inside) if you are still worried. However it is now almost spring and if he were going to suffer from the cold it would be apparent by now.
Knit him a rabbit coat, hat scarf and gloves.
Install central heating into his huthc.
Send him on a wintersun holiday.
Give him a job that involves plenty of manual labour.
Insulate the hutch.