First, get rid of the grain based and sweet feeds for two reasons:
A. Not eating supplemental feed eagerly is a sign of possible gastric ulcers and grains and sweet feeds increase the discomfort associated with ulcers.
B. Pony breeds are more prone to insulin resistance which makes them more prone to laminitis/founder and starchy, sugary feeds like sweet feeds or grain based feeds increase the risks.
Put him on a forage based diet---grass hay and a ration balancer would be the best choice. Ration balancers are concentrated sources of vitamins and minerals designed to balance out the nutrient profile of hays to meet your horses needs while being fed in small quantities.You should contact the company that makes the ration balancer you choose and ask about the appropriate amount for a mini as bag labels are for larger horses. Free choice hay is also important because if he has ulcers, having forage in his GI tract helps buffer the acidity. Then if you feel like you want more calories in his diet you can add a fat source--oil, weight builder product, etc. Whether or not he has ulcers, these diet changes will have you feeding a better diet for a mini to help minimize the risk of laminitis.
Have a talk with your vet about both gastric ulcers and insulin resistance. Diagnosis of ulcers can be made via endoscopy, but your vet may also choose to try treating and see if he responds without scoping him.