Excessive Drinking and Urination, Confused and Concerned


My wife's 10 year old EMS/IR gelding Mustang, Shiloh  has been drinking and urinating A Lot! for the about the past 6 weeks or so.   I read in the 'overview brochure' that this is a sign of Advanced EMS and PPID.
I guess I'm  confused/concerned with the arrival of a new 'advanced ems symtom' as I was thinking we were making progress.     Any Ideas on what might be going on ?    Looking for Feedback, possible paths to explore.

He was sore and lame with Laminitis some in 2018-2019, but Over the past year on dry lot with tight diet control and new vet/farrier he has lost over 200 lbs and is at what I would consider pretty much ideal weight (~870 lbs on weight tape  at 14 hands), and he's been comfortable and 'sound' for 9-10 months, athough we are proceeding cautiously with exercise at this point.   

DVM first impression is that he is just drinking a lot due to the heat, but I'm not sure about that. 
His only ACTH Test was in January of this year and it was 18.9 pg/ml.   (w/overnight fasting)
His Insulin at that time was 35Ulu/mi and Glucose was 98  mg/dl (w/overnight Fasting)

His latest Bloodwork:  Insulin: 21.58,  Glucose:93  (non-fasting.  although he did not have 100% constant access to hay and he did eat 1/3 lb of ration balancer about 1.5 hours prior to the blood draw)...
I know these numbers still indicate uncompensated IR, but hoping to get in the safer zone as able to increase exercise.

Daily Medications:
Metformin  (DVM is likely going to take him off this at next visit)
1 scoop Thyro-L
Prescend 1 tablet
Joe S. in TX 2020

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