Not intending to wholly disagree with previous post, but I would not automatically assume that the father is unable to transact business.
Do get a medical opinion; Early stage Alzheimers is not always that severe. Even with absent mindedness (and occasional odd episodes) an early Alzheimers patient can still have the mental capacity to deal with property. It is not that uncommon for an early Alzheimers diagnosis to bring about a situation where the sufferer himself says "I must deal with my affairs while I still have the capacity". Obviously, it depends on the father's current condition, but if he is still capable of living on his own it is certainly possible that he still has an adequate mental capacity to transact legal business.
Regarding the business of changing a will, this is a complex enough area. The standard for making a valid will is mostly concerned with addressing the isue of whether the person understands what he is doing: there is case law in which some guy suffered from insane delusions (specifically, he thought that his farmyards animals were consorting with each other in highly unnatural ways) but where his will was nevertheless held valid, because - despite the insance delusions - he fully understood what he was doing when he made his will. In any event, this will already leaves the house to your boyfriend, so it is fine as is.
One final comment - and please don't take offence; I have never yet seen a situation where it turned out to be a really good idea for the inlaws (or as in your case the potential inlaw) to get deeply involved in a family debate about wills\property transfers\care of parents. I accept that the mortgage (if it happens) will involve both of you, and that care of the father will affect you too. Nevertheless, tread carefully.