No. Most facilities, including Northwest Correctional Complex, do not allow stamps to be sent directly from outside. Stamps are considered items that can be used as currency inside the facility, so they are treated as a controlled commodity available only through the commissary.
The right way to get stamps to him is to put money on his commissary account. Once funds are on his books, he can purchase stamps himself through the commissary on his regular shopping day. That way the quantity is tracked and the system maintains control over how many he has at any given time.
InmateAid can help you find the approved deposit method for Northwest Correctional Complex if you are not sure how to add money to his account.