Anyone have suggestions on feeding a cushings horse? A horse I lease has recently been diagnosed and is on pellets (though I don't know what type) and he's losing weight. His owner asked me to do some research. Someone suggested that cinnamon helps but I'm not sure how.

Providing an appropriate diet for the horse is important. This will usually involve the elimination of simple carbohydrates.
eliminate all alfalfa and other foods rich in carbohydrates and sugars, as a safeguard against the laminitis.

Your best bet would be talking with your vet. Have a good exam done (possible blood work), then discuss his diet, and possible medications (these are easy to give and fairly cheap). They are developing some good diets for cushings horses. But definatelly pull him off the pellets if they are alfalfa or a blend. A bermuda or coastal type hay is what they recommend where I live. If the horse is losing weight you may also want to have the horses teeth checked and floated if they have not been done recently.

You can ask your vet about Nutrena's Lite Balance, we have an IR mare and we give it sparingly as she is on hay.
I would also ask about adding fat to her diet.

ADM has one for Metobolic Horses but I haven't tried it as the NLB has done well for our girls.

Most of the major feed companies have feeds specifically made for cushings and Insulin resistant horses. But as read above carbohydrates are a big factor. Which are most of your grains.
I have a 20 yr old pony that is on this diet and we have eliminated all RX medications[it works for her,yours may be different]
Beet pulp with out molassases-6 lbs total for the day,soaked weight
Complete advantage-Purina brand feed
Spillers Happy Hoof- Low carb. bagged haylage-theres carbs. in hay too.
MSM

You don't mention an age but they shouldn't get joint supplements. GLUCO[anything] Gluco =Sugars. Also now that spring is here be cautious about turnout time on grass.

As for the cinnamon, I just wouldn't, horses don't have access to it naturally, so there is no telling what damage it might do.
Chromium is a thought although new studies have shown it to be in-effective as well.