The only ideas I can suggest are for you to have a few family photos around that put you in context, e. g. We have a photo of myself and my brother as children which I refer dad to, e.g. Don't me and x look young in that Dad, I wonder how old we were etc. This subtly reminds him who I am when I think he's not sure. Also, take every opportunity to call him dad, eg. At the start of a sentence.