I used to do 1/2 at breakfast. It can be a pain to get them to eat it. We tried pony nuts in the hand which worked for a while, then half a slice of bread with jam on, then finally a handful of sweet, mollased mixed feed worked best. Something like meadowmix, the complete opposite to what you want to feed a native pony. But it got the meds in.
Our seasonal high got earlier, started end of November, then mid November, then end of october.
I would definetly bump up to 1/2 a tab through october and November. And I wouldn't bother with retesting until may/june. His levels will be higher because of autumn/winter. You knew what they were in august, test again in may, then again next august to get comparables.
My pony was only ever borderline, 90 I think was her highest reading in November. Mild rotation on xrays but absolutely crippled with lami, drinking loads and weeing loads. So the level isn't the best thing to judge how well they are, how well they are is best judged by looking at them.
Make sure he has plenty of rugs, mine really did feel the cold despite a massive rug and in at night from October. Also watch out for daft stuff like mud fever, runny eyes and lice. Their immune system is shot so they get loads of niggly stuff.
I know lots of ponies live long lives with the diagnosis but some shouldn't imo. I knew mine was ready to go when the rest of her herd started leaving her behind. And she was spending more time alone than with them.
You will know when the time comes, cushings isn't a death sentence but it is progressive and each autumn she got worse, despite meds.
Loved her to bits, but it wasn't fair to keep going and ahe wasn't even old at 18. But she started looking old. This is her in the spring as we pts in august.