Some polyamorists are former cheaters, yes, but not all or most.
If he won't listen with his ears maybe he will with his eyes. Write him, pen & paper or email, and lay it all out there, let him have the time he needs to process what you said. Invite him to ask questions about what you write or write you back with his own feelings and questions.
Communication doesn't always have to be verbal, it can be touching or writing as well.
Set up a calendar (I wholeheartedly endorse google calendar for this) with both of your schedules on it & write in a different colour when you will be available for time with him. Have him write in when his dates are so you can schedule something else for that time frame. Try to get to know his others so you can get a sense of who they are, you never know, you might find a good friend among them.
Go to www.xeromag.com
and read what Franklin has written. He, and many on this forum, have awesome insight and words of wisdom.