This is a tough one. It may just be that you can’t. Seven means his collected walk is “fairly good” after all, that is, without major flaws. Obviously the score implies that his rhythm is pure, and the “activity” comments indicate he’s putting himself into it.
What makes an 8 is the swing in his back and even more elasticity than what he has already. This comes from enhanced throughness–lots of transitions through lateral work are a popular prescription. His “bearing” is also a factor. Riding his poll up and out so the collection doesn’t restrict his shoulder freedom will maximize whatever amount of scope he has. Realistically, horses simply have a natural limitation to their walks. Anne Gribbons says your main duty in training the walk is not to screw it up. It doesn’t sound like you are! In the context of what you’re already doing, remember to follow enough with your elbows and hips to give him his best shot.