Your position is stronger than you think. A buyer will want your cooperation as a top priority. The owner knows this, or will soon find out. You don’t need to persuade him so play it cool.

Definitely talk to the owner. Maybe start by asking him, how does he expect the process to work? Will you be expected to continue working on the product? (play naive). Then let him talk, see what he’s thinking.

Best to be calm. This is your strong card, but keep it low key. This will emerge during the sales negotiations. The owner is likely to present you with a new contract to sign during the sale process, and this will be your moment to negotiate a fair outcome.

And you definitely want advice from a lawyer, starting now. At a minimum to review your existing contract.