I believe Young Jolyon knows exactly what Bosinney's intentions are and doesn't know quite what to do. He doesn't want June hurt by a broken engagement, but at the same time he doesn't want her to suffer through an unhappy marriage--the sort of marriage he had with her mother. He can't express this to his daughter or his father. He's talking to Bosinney as someone who was in a very similar position and chose to end his marriage, and Bosinney knows this. Jo's a Forsyte, he can't just tell Bosinney "Look, son, you'd better throw my daughter over right now before the damage gets any worse. Good luck on that thing with my cousin's wife."