Thanks for the update! Been reading up about this since you had originally posted, as it was something I had never heard of before. Believe me, it can get scary if not tended to.
With the pictures, I still think they look like summer sores/Habronema. I strongly suggest you google it and research it yourself a bit, I think you will be sufficiently convinced, as well as disturbed enough to really want to get on the situation and take it seriously. Perhaps send some of the veterinary articles about them to your vet if he isn't familiar with them.
Just because your horse has been recently dewormed doesn't really mean he can't further be exposed to them after the fact.
Also, from what I have read, left untreated, they can go away on their own and reappear again time and again, year after year.
Action to take recommended by the websites I saw are as followed:
Aggressive deworming with Ivermectin
Application of an anthelmintic and an antibiotic directly to the sores
Fly management and control on the property
I have also read some people suggesting putting ivermectin directly on the sores
One article I read was about a donkey getting them every year, the donkey was located in Brantford, Ontario - not sure how far that is from you but I'd suspect it is reasonable to assume it's something that is in your area.