I don't try to do everything. In that situation I would just tell him to finish up and that I need to make dinner. For the big topics of interest I've tried these:

1. Can you make some illustrations/create a time line? I need more visual aides.

2. Why don't you record what you want to say and we'll email it to your (insert lucky relative here)! Then you can call them and talk about it.

3. Can you make up a fictional story with illustrations that teaches this concept? Then you can read it to your brother.