A persons medical history has nopt relevance to a mortgage application what so ever and will not affect any lending decision. If your hubby's credit and income are good enough then there will be no problem adding him to the mnortgage.
I think what you are concerned about is protecting your mortgage through life assurance. Cancer will obviously have a very significant affect on any decision by an insurance provider whether they will accept the risk his cancer may pose. Different providers will have different views on how and what they will accept. If he has had a number of clear scans they may accept the risk but do not be surprised if they ramp up the cost of the policy to accomodate the risk. Also be aware that they may even refuse to accept the risk until he has been clear of cancer for 5 years.
There are providers that will accept the risk if the mainstream providers refuse it but be prepared to pay very high prices to have the cover.
Speak to a mortgage broker and he will be able to assess which provider will be able to better accommodate your needs. Do not speak to a banks mortgage adviser as they will only offer you their tied partner insurance and not assess your needs specifically.