It could be the formula switch since the stage 3 formulation can sometimes be a bit different from stage 1 and 2, but it could also be heat rash or insect bites given the location on the feet and legs. I would weigh both possibilities before assuming either way, so it might be worth keeping a simple log of when the bumps appear and whether they seem to follow feeding times. That said, since he is still quite young and the rash appeared around the same time as the switch, it is a good idea to bring him to your PD just to get it properly checked out and rule out any allergy.