It looks like 蚤咬 more than 疹 to me. Check the baby's bed / area he stays most of the time. Maybe you can try those "Bi Go" flea killer. It's very easy to use and very effecient, but make sure you read the instructions carefully and bring the baby out with you for couple hours. Hope he feels better soon.
This doctor is just a limited company as even this obivous skin rash is simple and common.
It is not the place to ask for diagnosis and treatment of disease on the internet, especially concerning disease in small child.
You never know which is the correct answer and whom is the people(medical knowlege and liability)answering you.
it is no point to save money by doing so,or just the correct diagnosis is delayed and the condition worsen. More suffering of the child is certainly the consequence