I get some boxed ornaments for kids to decorate, I usually use the foam ones, only I buy googly eyes, felt, markers, sequins, anything kids can add to make it theirs. My grandsons loved it. We still do it and they're 12 & 17.
I just finished a wreath with my 6 year old granddaughter. I fashioned a metal clothes hanger into a circle then cut old bread bags into 3 x 7 strips and had her tie them onto the circle. when it was full all the way around we fluffed it out, lightly spray painted it and hung it on the door. she was so proud of herself. maybe this will help you with a project.
One of my all time favorites is the picture puzzle pieces wreath. It can be as big as you want to make it. I really like the small tree ornament size with a school picture of your grandson or family individual pictures. You can hang them on the tree or hanging off ribbons hanging from an old/vintage (made to look) frame.
Make a toothpick tree! Get 16 styrofoam balls about 1.5-2" in diameter and 16 boxes of 100 count round toothpicks . Stick the toothpicks into the balls - 1 box per ball. You'll need a base and the best thing for that would be a styrofoam ring about 10-12" in diameter. Stack the "toothpicked" balls onto the ring starting with 7 on the bottom, 5 on the next level, 3 on the next level, and 1 on top. When I was a kid I used spray snow to finish it; but you could paint it with any color(s) spray paint you want and any ornaments you desire. They do make little ornaments that would fit or you could just make your own. If your grandson has a great imagination - let him go nuts!!
One year we make costumes out of paper bags (santa's, angels) and the kids wore them for our holiday feasts. Hometalk has so many ornament ideas that are really cute and easy. Just type xmas ornaments in the search line.