Hello,
If your son is having the bump on the scalp and his fingers as well after picking up the scalp one it can be due to plain wart caused by HPV which is contagious. You need to get it examined from his pediatrician to confirm the diagnosis.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
Possibly Verruca vulgaris. Should disappear in time.Do consult your doc.