Jackson is very relaxed and balanced when you're asking him to do things. This is really nice to see. Also bitless...wonderful!
You have a really great Spanish walk. Improvement comes slowly, over time, through building the strength in the haunches, and gentel stretching exercises to help limber the shoulders.
Collection is built also over time, and Jackson is still young, so don't rush it.
Oh, and what we do here might look like trick training, but it's not
We follow the concepts of classical dressage, without bits or spurs and precious little pressure, for the mental and physical well being of the horse. The horse comes first, always. AND is a philosphy.
We learn from the techniques of the old masters, but in a more humanitarian way, where the well being of the horse is considered before all.
All that said, you can being to teach Jackson collection and stretching exercises by wandering through the groundwork stickies, learning and applying what you learn there.
All the exercises are interconnected and they all build strength and ability in the horse.
So what I'm saying is, that Jackson is doing great for his strength level. If you can practice other things (mostly in hand) like ramener at liberty, from a pose into movement, lateral work in hand (great for building strengh and extension), Goat On a Mountain, bowing, and placing one foot on a pedestal (then lowering the head), all at the horse's own pace, then Jackson will continue to grow in strength and continue to enjoy is time with you.