Just because he has 2 relationships at a time doesn't mean he has only ever had two partners. I assume he meant his non-live-in partners, even though there is only one of them at a time.
Lots of mono people will say "I have done this with my boyfriends/my girlfriends have all enjoyed this" and that doesn't mean they weren't mono.
Lots of mono people will say "I have done this with my boyfriends/my girlfriends have all enjoyed this" and that doesn't mean they weren't mono.