He looks like a very pretty arab pony from that head shot in your avatar. Remember that it is not healthy to allow horses to become too overweight or overweight long term & that he is probably a fine build too.
Horses should get a bare min of 1.5% bwt daily in roughage(assuming dieting) & more regularly around 2.5-3%. Their system doesn't cope well with periods of hunger so this needs to be fed so the horse gets near constant small amounts. Eg. you might want to use a slow feeder for a gutsy pony.
Horses don't cope well with starchy/sugary diets and this also makes them more prone to gut probs & laminitis. Therefore it's best to feed low NSC hay(not 'improved' cattle fattening varieties) but if the hay is high sugar, it can be soaked a while in fresh water & drained to leach out some of the sugars. Because high starch diets aren't great, I would avoid grain & such unless absolutely necessary & no better alternative.
It's best to feed whatever little & often, as their stomach is small & doesn't do well digesting big meals. This is especially important if feeding anything starchy/grainy, as big influxes of undigested starch to the hind gut are a big problem. It's also especially important if feeding oils/fats because horses don't naturally have the enzymes to digest them & little & often is necessary to produce these.
So... can't tell you whether '2 flakes' twice daily is adequate or not, not knowing his or the hay's weight. If he needs extra 'groceries', I would personally start with more hay, but if 'hard feeding', choose a low starch, high fibre alternative to the Patriot Performance feed(Wheat middlings being top ingred, ground corn being 3rd is main reason & seems high octane) and feed it over a couple more meals daily if possible, or at least feed a fair bit less. I'd also be adding a good quality nutritional supplement appropriate to balance his specific diet. FeedXL.com is a great resource for all that sort of stuff.
Oh, keeping horses cooped up is not good for their health or wellbeing(I appreciate sometimes unavoidable or necessary due to injury), so I'd get him out of the stable & into 24/7 turnout ASAP. *As per DRichmond's advice tho, be careful about too much/too rich feed tho. I'd say regardless of breed/type.
) in circles is hard on a horse's joints and mindless circles for the sake of exercise can be hard on a horse's attitude, so I'd personally keep this to a minimum(I use 'lunging' type exercises as a training tool). Instead you can take him for walks, ground drive him, etc.