He does sound a bit selfish to be honest and needs to realise that you need and deserve attention too. It sounds as though you are doing all the giving here and recieving nothing back.
To answer your question about girth. No, I don't believe that that changes with aging. I'm mid-fifties and my length and girth is the same as it has been throughout my adult life.
What can change however is the lessening of the stiffness of erections which may give the impression that girth is reduced. As pps have suggested this is what may be happening here.
Lifestyle especially has an impact, as I guess you realise. Whilst a younger man might drink, smoke, do no exercise and not have any erectile difficulties it's not necessarily the case with an older man. Being as fit and healthy as possible, having a good, balanced diet and getting plenty of sleep really is the name of the game. His depression may well be playing part too which will need some medical support if he's not getting that already.
Getting on top of these issues may well resolve things, if he is willing to engage seriously. I realise this is easier said than done for some people.
There is also the option of ED medication eg. Viagra (Sildenafil) or Cialis (Tadalafil) which is available on prescription online or over the counter at a chemist. Occasional use of these medications (speaking from personal experience as a 50 something year old man) can be very effective indeed.
But first and foremost I'd recommend encouraging him to improve his lifestyle - a good thing anyway - before considering medication (ideally after direct consulation with a doctor), as this will probably go a long way to resolving the issue.
But also, crucially, as I said at the beginning getting him to understand your needs and his responsibility to focus on your pleasure in the bedroom at least as much - if not more - than his own.