My son was about the same age as yours last Christmas. My trees are full of breakables, but I put the ones that really meant something out of his reach. We continually said "Pretty tree....NO TOUCH", and that worked. I knew it really go through to him when we were driving around looking at peoples Christmas lights and decorations and I hear him chime in from the backseat "Pretty tree...no touch!". :) We didn't have a problem. Not that he didn't try to investigate, but we were pretty diligent in letting him know that he couldn't touch it.
My son enjoyed seeing The Polar Express last year. This year we are going to go to Huckleberry Railroad to ride their Christmas train and see the lights...I think this is a great tradition to start for our family. (My son loves trains.) Growing up, we would get together with family friends on Christmas Eve and the parents hired a Santa to come for all the kids (parents supplied a gift for Santa's sack, and we all got to sit on his lap and tell him if we were good, and what we wanted for Christmas). Mrs. Claus was with him and it was a very nice tradition. My parents actually are doing the same for our kids now (but not on Christmas Eve, as there are new traditions for that night).
You may want to do a small tree in your son's room for his ornaments. I still tell my son it's for decoration and he shouldn't touch, nor is he allowed to plug it in (that's a given!)...but it's something he can help decorate and enjoy his special ornaments.