This may be atopic dermatitis or a sweat rash or miliaria. These are noninfectious skin conditions that affect infants .These may be readily resolved using topical medications. I understand you are worried about this. However, in the absence of fever, irritability and loss of appetite this may not be something your should be worried about. Just wait for your physician's complete assessment before applying any creams or lotions in the area.
We went to his doctor today, and she said that this is some kind of naevus; some kind of birth mark...but it is showing up now, not when he was born. Some blood vessels patches, that will disapear over time and that it is nothing to worry about. I hope that they will go away as she said.