Can you share a current picture of him, so we can see what he looks like? Do you have a current weight for him? Did the vet just eye-ball him and say he's overweight, or did he take some sort of measurement? [My previous vet (he retired), pretty much just assumed all minis were 500# (at least he did when he was telling me dosage for bute for a laminitic mare, good thing I knew better, and adjusted her dosage for her actual weight), they aren't. There is a formula in the LB info pages that is pretty good for figuring mini weights:
http://www.lilbeginnings.com/info/misc/ There is a chart and a formula on the page, the formula is usually a bit more accurate. With minis, weight tapes are only good for keeping track of changes in weight, not so much their actual weight.]
The suggested amount to feed horses (big and small) is 1.5%-3% of their bodyweight in feed per day, most of it should be forage (hay, pasture, cubes, etc); the 1.5% is usually for easy-keepers or fatties on a diet (at least til you get their weight under control), 2% is usually for average horses, and above that is for hard-keepers, hard workers, and some broodmares (lactation can take a lot out of them).
I have mostly B-size minis, my three 38" mares range from 380-405# and actually look pretty darn good, although each could probably stand to lose a few pounds; my 36" gelding is 385# he is a bit chunky, his 38" gelding friend is 360# (he has Cushing's, so a struggle to keep his weight just right, he looks best at about 350#); my three 36" horses are 300, 330 and 340# (the lightest is my stallion, he's very fit). [i also have a 36" mare with Cushing's, she's at 300# and needs to gain, but again the Cushing's makes it harder to keep her weight just right). I'm in the middle of revamping my feed program, but the forage part doesn't change much, they typically get 1.5-2% of forage per horse per day; so... Depending on the horse (and weather), that'll range from 4.5# to 8# per day. [Right now, the mares get 1-3 hours grazing per day, 1# grass hay pellets per day and a couple pounds of grass hay per day. I'm about out of hay, so that's why they are getting the grass pellets (they'll help stretch the hay I have left, so I can make it to new crop). The two geldings graze most of the day on so-so pasture.]