If the cavity was deep then yes, some sensitivity can be normal. It may wear off after a while. Some dentists will put in an "insulator" between the tooth and the filling if the cavity was deep, this usually cost extra and may not be covered by insurance. It also may be the case that this "feels weird" to you son, he may need time to adjust to it.
If it persists I would recommend taking him to see Dr. Janda.
Janda, James M DDS - Prairie Point Dental Inc
Address: 3015 E New York St # A10, Aurora, IL 60504
Phone: ###-###-####
He has two offices, one in Naperville and one in Aurora. He is increbibly gentle and we all love him. As an ex-dental assistant I can also tell you that he is an awesome dentist. It never hurts to get a second opinion. =)