my dad was something similar, but not nasty, just didn't want to be a 'bother' to the doctor. so i took over, rang the surgery and arranged a house call without telling dad. i was there when the doctor came and the upshot was the doc pursuaded dad to go into the hospice 'just to get his medication sorted out'.
you can arrange a house call for him yourself, you don't have to go into great detail about how your dad doesn't know, etc. of course, it's up to your dad whether or not he speaks to the doctor once he's in the house. worth a go though as once the doctor has assessed him he'll be able to put other agencies in place if necessary, sounds like your mum could do with some support.