Most Helpful Guy
You can choice to ignore the messege and maybe respond at a further date when you feel more comfortable with an explaination for the slow response.
Be it honest that you still felt too hurt to reapond or dishonest that you did not see the messege and your reponse was fast given you just noticed it.
Whatever the reponse best of luck healing after the breakup to you.
Most Helpful Girl
I would be honest with him, there is nothing worse than having someone just ignore you. I would tell him honestly that you arnt sure you can be friends with him yet, that you are still hurting over the break up and unless he wants to try and work the old issues out, you cannot be friends with him til you are OK with how the relationship ended.