Hmmm...
So I've been in a similar position a few years ago with my app. I had made it in my free time and it took off. Not like this guy, but enough that the demands placed on me from users were quickly consuming a lot of my dwindling free time. When I started the app, I was married but had tons of time to work on it. Within a year, I had a newborn and too much going on in life to keep focusing on the app.
So what do you do? Well, I approached the company I worked at to see if they would "invest" in the project... aka, let me work on it at work. That never really panned out. I then brought my wife on to the project to help with email and support (she had lost her job at a school district after our son was born).
Soon, the whole thing turned into a giant intellectual property mess where my company was claiming it was there's and... ugh. There were personal issues at stake and friendships got damaged. I just wanted to "kill" the app so that life could just go back to the way it was. Money creates problems and some people's dark sides come out when the green shows up. Once there's money coming in, and things get popular, everyone starts staking claims. A lot of developers are not equipped to deal with popularity, relational conflict, money issues or legal threats. So, he's probably reverting to "default" which is to "kill" it.
I think the most plausible explanation is something legal coming up that's "the last straw". My advice would be to partner with a third-party company to cover him legally and allow him to do what he wants and handle all the other stuff for him. But again, it just depends on him and he may just want to do things for fun and not this mess.