Shop around with other life insurance companies, he will have to declare that he has already been declined. Some companies look at illness risks differently however he may find that he is unable to get insurance at an affordable price.
If the mortgage lender he was applying to is one that allows a waiver in this case, as some do, then he can still go ahead with the mortgage. If not, then I think the only option if he still wants the mortgage is to switch to a lender who will accept a waiver in the case of declined cover or expensive cover.