you don't need obligatory a confirmed return flights - but you may provide other (written/printed) reasonable proof that you will leave the country not later than 90 days after immigration.

I very seldom (but it happens) have been asked to show my return ticket at immigration (but always how long that I intend to stay: in your case I would than answer: we will leave not later than ...) - but always at check-in for the transatlantic flight from the airline (and for them an open ticket should be just as fine as any fully exchangeable full price return ticket.

Such proof could be, as an example, printed hotel reservations outside the USA, etc.