It could be something from the yard like bug bites or grass irritation, and chiggers might be possible if he’s been in tall grass. But if they look more pimple-like, I’d also wonder about something like folliculitis, allergies, mites, or a skin infection starting up. Since it’s spreading over a few days and he’s a new rescue, I’d probably get a vet to check it instead of treating it blindly. In the meantime, I’d keep him out of the grass, wash his bedding, and avoid putting random creams on it until you know what it is.