Puppies should be supervised at all times. She or he should be crated while alone, leashed while you are around. Time to teach the “leave it” command. You will really mean it when you fear for the dogs safety! Baby gates might deter him – unless he’s that curious kind that looks to defeat all confinement. If he’s leashed it’s easy to give a sharp leash correction if he goes near the tree. Put the tree up for a few days without trimming to get the dog used to it without risking fragile ornaments.