Your needs and your son's needs are clearly not in agreement. You tell him he "needs" to take care of his beautiful room. Well, his need is to have a usable space that's comfortable for him and allows him creative (or, as you see it, destructive) options.
It would be fair to tell him YOU need him to keep it as you designed it. It's not really fair to tell him what he needs to do. That won't ever make sense to him on a deeply emotional level.
But how about offering him some creative alternatives? You could, for instance, put up a big sheet of butcher paper on one wall, and let him make whatever art or murals he likes. If you hung a roll of paper against the wall, old scribbles could be torn off when he wants a new surface. A big bulletin board might give him other options. And be sure he has plenty of physical activity so he doesn't need to climb the walls. (Or what about a climbing wall?)
His paint alteration sounds potentially creative in its own way. It's not actually his fault the paint peels so easily. How about a camo paint job that he could help with and take pride in? Or a jungle theme for the dinos? If he has a say in how his room is decorated, he'll be more invested in keeping it up.
I was born with a crayon in hand, and drew on the walls from a very early age (I now do illustration for a living). I got in trouble for it often, but I'll tell you, there were the most hideous duckies and lambies with pink cheeks and smiling lips all over my walls, and even at a very young age, I could not stand them. The crayons seemed my best available option – I could put in pictures I liked better. Of course, my mom didn't care that I didn't like them, because SHE did, and she put them up. Nobody asked me what I wanted.
I hope you and your fiance will read the most wonderful and wise little book How to Talk So Kids Will Listen, and Listen So Kids Will Talk, by Faber and Mazlish. The book is filled with real-life examples of how parents help their children become creative problem solvers on all sorts of issues. This approach is amazing with my 6yo grandson.