You say he "hired" you, meaning you were paid to take the portraits. If that is the case then adding on an additional fee for additional requirements shouldn't be too much of a problem. Just make sure you provide an update usage license that clearly states that the digital files may only be used for the printing of the year book.
Obviously if it was originally done for free/as a favour, you now have a case of friend creep; where a seemingly simple favour become more complex as time goes on to the point where it is no longer reasonable for them to expect it to be a favour/free but hard to bring up the topic of payment.
I am learning photo graphee - see the results at www.danmarchant.com