Abby - the cheapest option for your husband will be to get a travelcard - weekly, monthly or yearly. Obviously the longer ticket he gets, the more it will cost, but the more it will save him. It can be put on an Oyster, whihc is kind of like a swipe card, which will make it quicker going through the barriers at rush hour.
MrsP - I am pretty sure there is no expiry day on pre-pay. It is certainly worth getting, even if you are only in London ocassionally. A single tube ticket in zone 1 costs £3 - on Oyster it is £1.50.
Some rail companies do accept Oyster on parts of the lines.
Victoria