It could be the formula switch triggering a reaction, but it could also be something else entirely like heat rash or insect bites, so it is hard to say for sure without a proper look. The fact that it showed up around the same time as the switch is worth noting though, and I would bring him to see the PD just to be safe. The PD can help rule out an allergy and advise whether you need to switch back or try something else, which gives you a clearer picture rather than guessing.