It might be ok if you just said "I love Mono the same way I love your father, but it doesn't affect the way I love you. I love you the MOST."
My mother always told me that, and she was seeing a married man for 14 years (it was an "affair" his wife "knew" about, but it wasn't "poly").
But some mothers get all up into explaining how "There are different kinds of love, etc. etc." Kids that age, or any age really, don't need to process all that. They just need to feel secure that their world isn't going to come crashing down around them. Just tell your son that whatever happens, you (and his father) love HIM the MOST. It's probably true anyway.