nodes without any other symptoms are normal finding in children. As children grows these lymph nodes regresses in size.
Enlarged lymph nodes may also be due to some local infections.
In case of your son he is asymptomatic and there is no sore throat
infection. So it is just a normal finding. Your doctor has advised him correct. So you just be watchful for any change in swelling like pain, increase in size etc. and don't worry about this.