Solution? Give him his own tree!
To make a flannel board Christmas tree, all you need is felt and fabric scissors. NO SEWING!, and almost no time to make, either! Just cut out a basic green tree, and then some simple ornaments from different colours. Easy-peesy!
I set this up for him while he was napping today, and when he woke up, he was so excited. It may as well have been Christmas morning.
He picked off ornaments, and switched them around, and held them up for me to see for half and hour straight. It was so great. And it was an unexpected way for him to review his colours too. He kept choosing an ornament and asking me, "purple?" "green?" and I'd tell him which colour he was holding. It was so fun and so cute.