I'd tell him you were happy to help, but your session is your time to talk about your problems, and you would appreciate a credit/refund for the two sessions where you did not discuss your problems/issues. That sounds cold, but it's what I would do and be perfectly within rights to do so.

Tell him you are more than happy to help him, you would just prefer not to PAY HIM to help him. That's hardly fair.

I do think its ok you helped him tho, just not that it was done during a paid therapy session.