Personally havent gone through this myself but have known chefs who have severed their fingers and have had them reattached.
I would all depend on how badly damaged the nerves and tissues are. If they are able to repair the nerve and supply enough blood circulation to the site he should regain function of his finger. All depending on how badly damaged it is. Was it a clean cut? Not a doctor nor a surgeon so guess you will just have to wait it out. Good luck!!! Hope the surgery goes well.