While he'll be giving up a lot of wealth, it's unlikely that any meaningful NDA will be applied here. Maybe for products, but definitely not for their research.

There's very few people who can lead in frontier AI research domains - maybe a few dozen worldwide - and there are many active research niches. Applying an NDA to a very senior researcher would be such a massive net-negative for the industry, that it'd be a net-negative for the applying organisation too.

I could see some kind of product-based NDA, like "don't discuss the target release dates for the new models", but "stop working on your field of research" isn't going to happen.