I agree--you can get creative here. I make my kids' costumes and they have gone as He-Man and Orko, knights, and this year we're recycling He-Man and my 3 1/2 year old has asked to be a dragon (which I still have to make, but I'm pretty sure I'll be starting it with a green sweat suit).
You can build great costumes off of sweats and cheap tops with a glue gun or crazy glue, or even a few well-placed stitches with sturdy thread. If he wants branches, his arms will work and you can stuff old stockings with wadded brown paper (like paper grocery bags) and make twisted brances and attach the leaves to those. Just attach the stockings to his shoulders and upper body. You can use an old winter cap to cover with leaves for the top of the fall tree (either real leaves, paper leaves, or cut out felt leaves). Sounds like an imaginative little boy!