he did joke about how he had underestimated it
I imagine if that conversation had been had in person it would have been accompanied with nervous laughter.
I agree with minimum_wage. Sounds like he bit off more than he can chew and now he's nervous about the money side of things. There's skin in the game now and he probably feels morally obligated to deliver. If it's just a kick-around project then there's no real concern about it tailing off and scrapping it, but the money is a game changer.
Hopefully he reaches out and discusses concerns.