How to best utilize a Senior EM's mentorship?

First of all, congrats! Having an M2 support you is huge, especially for E6 promo. A lot of E5s hit a wall, because they don't have a couple people at this level vouching for them in performance calibrations.

What's his incentive for doing this?

1. He thinks you have a lot of potential and are a good person in general.
2. He is a very nice person himself. Are they helping other E5s that aren't within their org? If so, maybe they just really like helping.
3. While it won't be as much as an M1, and M2 should still receive a solid amount of credit growing an E5 to E6.
4. They are trying to raise you into a manager and have you report to them, expanding their scope.

How can I best utilize this time?

A lot of good resources here 😊

1. "How do you get the most out of mentorship?"
2. How To Effectively Leverage Skip Level 1:1s
3. 1 on 1s Are The Time To Be Awkward

Are there things that M2s are uniquely qualified to provide mentorship on?

2 things come to mind:

1. Growing mentors - An M2's job is to manage people who grow others (M1s and M0s). This layer of indirection is crucial to scale yourself, even on the IC track. You will likely need to do this as you grow to E6. It is very common for E6s to mentor E5s on how they can mentor E4s and E3s. Something you can consider is mentoring an E4 on how they can mentor an E3 or an intern.
2. What E6 scope looks like - In order to get to M2, you generally need to have a couple E5 -> E6 promotions under your belt. You can ask them for feedback on project ideas you have and see if they're Staff-level scope. You can also simply ask them what makes the E6s in their org - Why are they unique? What's their engineering "superpower"?