I do not think it is coincidence the bump is appearing exactly where the shots were given. I would look into this further as far as a reaction to the shots AND take him to a dermatologist. Some kids just can not handle the added toxins in the vaccinations. Their systems are so young. I would really think twice about continuing on the current "recommended" vaccine schedule until you know more about this bump. Dr. Sears has a book called "The Vaccine Book". It is and EXCELLENT book. It is NOT an anti-vaccine book in any way shape or form. What it does, is educate you on what exactly is in the vaccines, potential side affects and the risk/benefits of the vaccines vs. the illness. It allows you to make an educated decision vs. just taking the doctors word for it. Most doctors are not really aware of what is in the vaccines. In addition, many doctors do not look at your individual child or situation, but would rather follow a "one size fits all" type of medicine. Educate yourself A.. You would regret it if your son had a reaction and you were not fully aware of the potential side affects. WE are our only advocates when it comes to medicine. Doctors DO NOT know everything.
There are many out there, like Dr. Offit, who want us to believe vaccines carry NO risks whatsoever. That is just not true. There ARE risks. And, granted, some may be rare...but I was once a 1 in a Million statistic with a reaction to a medication. So, even if the chance IS 1 in a Million.... you or your son could be that 1. I was.
As already mentioned, the dermatologist can help determine if it is a cyst under the skin or something like that.