Ex doesn't care that our son has 2 girlfriends

My son is 13 and in the 6th grade. 6th grade hear is technically middle school, however years ago a lot of middle schools closed down so they put 6th grade in elementary schools. My son gor the past month has been talking about his girlfriend, but I recently found out that he's been dating 2 girls at the same time. I told him that's wrong and I would have his father talk to him about it. I told his dad over the phone and he for real said "This is too stupid for me to care about". I told him I don't want our son thinking cheating is ever exceptable and my ex says elementary school relationships aren't real and if he doesn't talk to them except for at school than they aren't real boyfriend and girlfriend and it's stupid to get involved. He thinks they're just friends with extra steps. It became an argument and he said "He hasn't even kissed a girl. This is stupid elementary school nonsense. I'm not involved". I don't think I'm wrong.

@Carlee I think it's a male vs female mindset too. I understand my sons idea of dating is childish. To him it's eating lunch with a girl and playing Pokémon go at recess and that's it. But I think respect should be taught at his age. My ex said no girl is gonna grow up hating men because her 13 year old bf pokemon go'd with another girl but honestly people have feelings.

@Jess THANK YOU! And I did try talking to him but he kept saying he doesn't want to play with only one girl. I thought his dad would be helpful but nope