FWB positive test!

We don't contact each other unless we're super horny and want to meet up. I want to sit down and talk to him. How do I tell him? He is always too busy and won't meet in public. We only meet each other for sex. Basically, the only chance I will have to tell him is through the phone. But I want to have a conversation with him about this and deal with this the right way. I want us to be on the same page and I can't communicate that in a text message.