If the opportunity still exists for you to tell him, then you can say either "Now I remember what it is..." OR "Now I remember what it was that I am supposed to tell you.
If you the oppotunity to tell him is no longer available, then use was for both.
For example: You coworker calls from the airport and tell you the boss's plane just landed, but he wasn't on it. "Oops," you say. "Now I remember what it was I was supposed to tell you. He got an earlier flight. He's probabaly been waiting for you for about two hours now."