'Tis the season to find this in our horses! Keep calm and do not get overwhelmed reading all about the condition on the internet. 
The Equine Cushings and Insulin Resistance site (ecirhorse.org) is probably the best place to start learning more about the condition and ways to manage it. Otherwise, some practical suggestions:
Prascend can cause a whole host of side effects that include lethargy and poor appetite. Many horses tolerate it better if they are tapered up onto the dose their vet recommends rather than starting it all at once. Talk to your vet about this- some taper protocols start with just 1/4 of a pill, some with 1/2. “Pergolide veil” can occur even in horses who have tolerated a dose for several months, so don’t be shocked if you start seeing it later on.
I feel like a broken record about this today, but a liquid adaptogen supplement called APF has given many of us good results in combating inappetance. If I were starting a horse on Prascend for the first time, I would just buy a bottle and start the horse on APF at the same time, too. YMMV.
The things he eats in his feed dish look appropriate. TC Senior is suitably low-NSC. Omega-3 fatty acids are associated with improved blood glucose regulation and can be helpful to horses who have Cushings. I have personally had really good results with marine-sourced (rather than plant based ex. flax) omega-3’s which were associated with much lower ACTH numbers in my horse. YMMV a lot on this one.
You should know that fall and spring are the most challenging times to manage a horse at risk for laminitis because warm days and cool nights increase the sugar content of the grass. Those little short stubs of grass are deceptive- sugars are stored in the bottom few inches of the blade of grass, so it doesn’t matter that it’s a short stalk. Do talk to your vet about this before you change anything- depending on the rest of his clinical picture it may make sense to restrict his grazing, turn him out at a different time, add metformin to help him manage insulin, both, or possibly neither. Muzzles are part of life for many but not all Cushings horses. Not a bad idea to pick one up just in case- GreenGuard and Flexible Filly get good reports. Soaking his hay can be another way to reduce NSCs in his life.
Heading into winter, keep an eye on his temperature- Cushings can interfere with a horse’s ability to thermoregulate, so you may find him needing more blanket or less blanket than he used to.
He’s gonna be okay- it takes some management and trial and error, but it is all very manageable!
