All the other advice is great and Id follow it - but do NOT use OLD glass balls at all - they can contain lead paint. If they break it's worse than regular glass.
I put the tree on a 4x4 5 inch high platform and use a big tree skirt as a physical boundary (no stepping on tree skirt - can't get close enough to touch). It worked okay.
What I found worked really well - he has as very small artificial tree of his own with safe ornaments. He can play with that all he wants, but only look at mom & dad's tree (we let our son set aside toys that are too special to share - and we say our tree in too special to share).
I found putting trains under the tree actually helped - he was drawn to the trains more than the ornaments. I also put a small wind chime on the bottom branch so the train would just barely touch it. He loves making the train go by the wind chime and hearing the tinkling of the chimes. Also, the trains are mine and I make it seem like a very special treat for my son or husband to use them. This really helps keep most of the ornaments on the tree.
Most of all, enjoy the season and seeing the magic through your child's eye. Children see the beauty and magic in Christmas without needing to have a perfectly coiffed tree. In fewer years than your realize, your child will be a teen telling you how to properly decorate a tree (as teens know everything).