Sudden hair loss around belly

[beth.e]

Hi, 2 days ago I noticed that my 14 year old gelding was loosing patches of hair under his belly. Yesterday there was also hair loss around his rib cage and this morning when I went down the hair loss was also evident on his back and shoulders.
A few of the hair loss patches have scabs from where he has scratched them but the majority are clean hair loss patches!
When I groom him the hair is just falling out!
I haven't made any recent changes to his feed or environment so I am confused at to what is causing this sudden hair loss.
I have attached some pictures so any advice would be much appreciated!!

 

[Auslander]

Fly spray? I have a TB here who loses hair if we use certain fly sprays on him

 

[beth.e]

Sorry picture attached

 

[beth.e]

Fly spray? I have a TB here who loses hair if we use certain fly sprays on him

I haven't used fly spray on him this year, I moved yards in February and there is a lot less flies at his new yard so it's not been needed

 

[PinkvSantaboots]

Could it be rain scald one of mine has a touch of it on his shoulder just in front of the girth line, it comes up looking like hives then the hair falls out, it's common in this warm wet weather we have been having.