Building a treehouse with my 7yo and it's actually happening

Started this project 3 weeks ago and honestly thought I'd give up after day one. Got the platform secured to the oak tree in our backyard and we just finished nailing down the floor boards. My son did like 60% of the hammering himself which was both impressive and terrifying. Planning to add walls this weekend. Anyone else build one of these? Tips on weatherproofing would be clutch because I definitely didn't plan that far ahead lol