Hi, I am now 65 years old and I too had a pea sized lump on my neck when I was a baby (back in the 50's) My mother was also very worried, but the doctor thought it was just a large gland and was nothing to worry about. I was born with asthma, so I visited to doctors very regularly and often had my neck checked. I believe he was right as I had it all my life and had not caused any pain or discomfort. However, in June 2019 I was diagnosed with prostate cancer and suddenly took a vast interest in my body! It was then I noticed that the lump was not there anymore.
I truly hope that what your son has, is nothing more than what I had.