Please help! My son got red spots yesterday evening, like bites, but …

Please help! My son got red spots yesterday evening, like bites, but nothing bit him. At first they were on his cheek, then on his hands and feet. The spots were small at first, then grew larger over time. I gave him fenistil as the doctor said. They went away at night, now they're starting to appear again. On top of the photo - initially small, from below - after some time they become big.