That grass strip sounds perfect for building topline however I would go about it a little differently. Get him into a nice big long trot, think of it like this: You know that long trot that a horse get into just before they start cantering? Well keep him at that pace, trotting. It is really hard work for him and builds great topline. You will know that you are at the right pace because his head will naturally lower as he has to really work to maintain the pace and truck on.
As an OTTB it will take a while to teach him that he has to maintain the fast pace at a trot as he will be accustomed to canter/gallop work but once you can maintain that solid trot pace, 15-20 mins of this a few times a week will work wonders.
As other said also with the long and low/circle work. Just a word of warning though - whilst flying changes are great to build the HQ, they require quite a bit of training to be done correctly. Even though most horses can swap leads at a canter, this isn't necessarily a flying change. The first step towards achieving this (also very good for building HQ strength) would be to perfect your upwards/downwards simple transitions. Then begin teaching balanced walk-canter and canter-walk transitions. Then lots of leg yielding excercises. THEN worry about flying changes!
Edited to add: He is beautiful by the way ;)
All horses deserve, at least once in their lives, to be loved by a little girl.