Yes, you can add him. Call Comcast and use the Billing department option. If you are lucky they will do it over the phone. If not, you may be told you have to do it in person at the local office if there is one in your area.
When I became the caregiver for my mother she added me as an authorized user on her account over the phone.
He will still be asked to verify the account information when he calls Comcast.
Comcast employees must be authorized to post in the forum in an official capacity. Employees posting here have their names in red and are designated as employees. Names not in red are customers.
This is done to protect customers and for assurance that they are dealing with a Comcast employee. Non-Authorized Employees are allowed to post but cannot state they are employees nor can they allude to being employees.