Is it an issue or is it just him getting old? It could just be the stress and possibly sleepless nights living with your Mum or there may be some underlying medical condition. My children say I'm getting more forgetful but I'm sat here at 4.45 in the morning and who knows when I'll get back to bed.
I said on here (a few days ago) that I hide the reality of the situation from my children to some extent I'd hate to think that they thought I was losing the plot when mainly it's just because I'm only half awake.
I do think you have to allow for the fact people do slow down as they get older and what you'd really need to do is look at how he is compared to people of his own age but either way if you have any concerns you could suggest he visits the doctors just to get checked out, there could be any number of health issues so an overall physical check wouldn't be a bad thing, as a carer the doctors may give him some more attention given as he (and you) are saving the NHS a fortune looking after your Mum.
K