Why not do both, at least temporarily? Take the job, and then when you're in a better financial position, quit and continue the startup.
Then #2, the industry my company is in is moving very fast and is very competitive. If I slow down too much, the company might get left behind.
So even if he would prefer you to focus on his thing, you could just state that working on your product on the side are your conditions for joining his company. And see what he says.
If he says no (and you feel he really means it, and you feel you really do want to work on his project full-time) almost certainly the 400k full-time offer will still be on the table.