Your picture is kind of hard to see. I'd say he looks OK, but take that with a grain of salt. It doesn't matter if you are going to show him or not. The point is that a horse needs back muscles if he is going to carry weight. You are going to trail ride - therefore, he will be carrying weight, therefore he needs muscles. The muscles will come with properly weight distribution and work. If he loses muscle for a little while, no biggie - it just means that when you are ready to ride again, you can't go all out right off the bat. But, you won't be ready for that either.
Back muscles are not for show. They are for health and well-being.