hello
@Mausie
a warm welcome to TP
as your husband has asked more than once and not coped with a truthful response, maybe you are right to consider another approach
if you know something of your husband's family's habits, maybe think of times that his mum was absent for a time and use that as the basis for a 'love lie' eg if she visited relatives and stayed for a while, say that's what she's doing, or gone on holiday, or off for a sporting activity or shopping spree - probably best to make the story a positive one, so he isn't worried about her (no hospital admissions) - be vague about when she may return or don't mention it at all - and counter any requests to phone her if he raises that
and maybe think of a reason why he's with you (just in case he thinks he should go back to his childhood home) eg as you're such a good family friend, his mum thought it would be nice for him to stay with you (not far off the truth, you are his best friend and anyone would agree he's best staying with you )
I was a bit hesitant in suggesting this, as you say your husband does not have a diagnosis - it's for you to consider what's in his best interests
I hope you have been able to share your concerns with his and your GP, to begin the process of checking out what is happening with your husband, as there are other causes of some dementis like symptoms eg vitamin deficiency