weird " pimple "

My son will be 3 months on April 13th and since he was about a month he got this one weird pimple below his cheek. An idea what it could be? It has stood around the same size since I noticed. The doctor said that he got it because that's around a sensitive area but that was almost 2 months ago. Could it be a birth mark? Any comments will help. Thank you 💙