Well, first of all I think in general he looks pretty happy!
Which is a good thing... Plus you don't look all stressed when he's being a "brat".
But he isn't working under himself. You can even tell just by looking at the hooves. The backhoves should atleast step into the tracks of the front hooves (it would be better if there about half a hoove over the tracks) in a normal working trot.
You have to work more, to get him to track up. Take your legs back a bit and use them ALL THE TIME. You should be giving constant pressur with your legs to make him round up his back, which will make him track up. I don't mean kicking him every few minutes. And I don't mean he has to go faster, you just have to use your legs constantly. It's a bit harder without a saddle, because you don't have the same stability for your legs, but It works fine all the same.
Hmm... hope that kind of made sence...
Oh and if you just want to trail ride and so on, it's not thaat important in my opinion that the horse always tracks up. Since your not fighting and he isn't going against the bit, tensing up his back, everything is just fine!