You aleady have him on a "nothing" diet, so that's good.
His gut flora is way out of balance and all that worming didn't help. I wouldn't be surprised if he didn't need wormed at that time
Yes, plan on keeping him on the Pre
-probiotic the rest of his life
I use Daily Start. One of their enclosed scoops daily. I feed it to two of my horses, so five pounds lasts a few months. You might find it cheaper than the link below.
Daily Start has Oligiosaccarides in it, which are the PREbiotics. Digestive Supplements for Horses: Daily Start Probiotic by Equi-Shine
Nothing is going to be 100% but this stuff does help. I know when Spring grass comes on, my Arab will once again have loose manure but it won't be as bad since I've put him on the Daily Start.
Not quite what you wanted to hear but I hope this helps