It sounds to me like he's actually pretty good at long-term commitments, so much so that when his relationships evolve in non-romantic directions he sticks with them and takes them as they are rather than breaking them off. Given the amount of societal pressure in the direction of breaking up and never speaking to people again, he'd have to be very committed to his exes to keep them in his life the way he has.

I don't know that he'd necessarily call himself polyamorous. I just think it would be useful for the LW to learn about different ways people put families together, as a stepping stone to seeing "my kids' other parents hang out with me and swim in my pool" as a perfectly useful way to do that as long as everyone involved is cool with it.