Your Vet should be able to give him something to calm his skin right down - a cortisone injection or similar. Here in France, the aoutâts/'ootah' or chiggers/harvest mites/berry bugs are a nightmare in August. I've had them so I've certainly felt sorry for my dogs. Fennel, you'll know what I mean.
If you look between your dog's claws, in the groin or behind the ears, it's sometimes possible to see a small gingery/reddish spot on the skin which will be made up of many many tiny mites, less than the size of a pin. They can cluster together sufficiently to make them quite visible on a dog - especially one with a pale coat.
I asked my Vet one year, when he treated my dog, if there was something he could recommend for me. Having admitted that I just wanted to keep on scratching, the Vet admitted he felt the same. But no simple, guaranteed. treatment for humans at that time. He provided our dogs with a special shampoo to be administered in the shower and that worked and got rid of the problem. Also a calming injection to start.
For yourself, don't sit on the grass and if you go for walks in the woods or any grassy spaces, put your clothes in the washing machine afterwards and make sure you dry them in a tumble dryer or iron them before putting them away. This does reduce any risk of them spreading. They're really miserable little biters for a few days when, if you see a line of pimply bites on your own self, they'll be in warm, dark places such as underarm, underboob, groin areas.
I've tried so many anti-itch creams and similar dog products for the dogs. but without much success. I think it's easier and safer to speak to the Vet, especially if your dog is, like mine, already on some regular medications.