Since they're in a cluster, my guess is he either rolled on an ant hill or got into some weeds with a cluster of young spiders in it and they bit him.
Unless they're oozing, I wouldn't worry. You can buy some hemeroid ointment (not the cream it's too messy) and slather it on them.
If it's ant or spider bites they should go down really fast. Some might scab over, others might not.
One of my horses came in looking just like that, on his shoulder, a few weeks ago. I slathered hemerhoid ointment on them and the swelling was gone in less than 24 hours.
That doesn't mean they couldn't be something else and you might need the vet but if he's eating, drinking, recycling all that, and is active for what is normal for him, I wouldn't worry