suggestion - next year why not use one of these sort of companies
http://www.mykidzart.com.au/
they can do a mug, a clock, a plate etc etc ...
you could also go to Plaster Fun House and let them paint a few little ornaments ... (do these places still exist?)
OR make a huge batch of White Christmas ... take cute photos of your son making it to incorporate in the card ... wrap some in cellophane with big Christmas bow and hey presto - handmade edible (hopefully) gift that keeps everyone happy
or take photo of him ... make a cardboard frame and let him decorate it however he likes (but preferably including the words I LOVE YOU GRANDPA!! etc etc and put it in a photo frame
OR there are calendars you could put his artworks on each month ... and print out 14 of them

include little reminders each month where there is a family member's birthday ...