If he only gets 2 flakes of hay daily, I'm presuming he's turned out at grass the majority of the time? If not, more hay and more turnout are definite necessities to start with. Free movement is important for a horse's digestive health, as well as other health & wellbeing reasons.
While depending on your scoop size, one each of sweets & oats daily doesn't sound like a huge amount, this could be the problem, along with how you feed it. Horses don't cope well with sugary, starchy feed and it commonly causes 'hind gut acidosis' which can include gas colic. The way their digestive system works, it's also important to feed little & often and high fibre, low GI. Therefore, if you continue feeding what you are, I'd be 'watering it down' with a fair bit of chaff and feeding over a couple more meals daily at least.
Check out safergrass.org & hoofrehab.com for some good info on diet as it relates to health.