Make pretty much everything insect repellent....sprays, shampoos, conditioners. You can also get supplements to repel insects. Use as much protection like fly masls and rugs. Aloe vera gel is amazing for soothing the itching if any bites do get through. You could even change the time of day your horse goes out, so for example, instead of turning him out during the day when there are loads of mosquitoes, turn him out at night when its a bit cooler and there's fewer mosquitoes
Try vinegar as an insecticide, it works on my horses... but the smell is not great 😓
For my horse, I put products on the hair to prevent this from happening... Anything that is repellent based on lemongrass ... for the eyes round essential oils of lavender for example. And that he has reactions! After that if it happens I disinfect the area where there was the sting (quite widely) then I let it dry and I put cream for the bites of mosquitoes, apaisyl (as for humans)
I hope this helps :)
Have you tried a fly sheet?
We make our own. And it sometimes works.