What's he like to hack out? Any different to the school work? It does sound like he's a bit bored with the schooling, but if he's the same out hacking, then it could be something else. If he hasn't been schooled much before he may not be able to concentrate for long periods and you may be better off schooling for very short sessions then recapture his interest by going for a hack. In fact, if you're working on basic schooling, you could do much of this out hacking and retain his interest. Or you could try introducing new elements into his schooling, like pole work or working him from the ground. With his breeding, he's likely to be quite bright and need constant new stimulation to keep his interest.
On the other hand, if he's showing the same behaviour out hacking, then I would definitely look at other possibilities. I doubt it's diet - but why on earth are you feeding him conditioning cubes if you want him to lose weight?! I'd suggest you switch to something like high fibre cubes or cool mix, or cut hard feed altogether and just feed good doer and a mineral supplement!
Have you checked his back and tack? If he didn't do much before you had him, he may well have changed shape with work (especially since he's lost weight) and the saddle may not be such a good fit - this would explain the sudden stops in trot and reluctance to go forwards. By the way, a horse with an uncomfortable saddle may only show problems in some situations - if he likes hacking, he may put up with the discomfort for the pleasure of going out. If he's bored with schooling, he'll be less tolerant.