If he is an old dog them i would suggest trying Activait which is a non prescription supplement for old dogs who might be getting dementia. The only sign in my old dog was that as he got a bit deafer, he would lose me round the house. It took a while to have an effect (I think they say six weeks) but it did perk him up and he was able to learn to go and look for me whereas before if he got somewhere and I wasn't there eg the garage, he would just stand and wait. he has learned some new cues to help him to find me, eg if the upstairs light is on and the downstairs loo door is open (he used to wait outside the closed door thinking I was in there) then I am upstairs whereas before the activait, new learning was not possible.
I've got another 'keen'... Ouch!