Yes, there are just as many if not more studies to say that 50/50 is very much not best.
But that doesn't even matter. Every child is different. Your partner's son is TELLING his father what he needs, and that this isn't working for him.
He would be wise to listen.
So was this his way of saying he wants to see his dad "less"? (All his friends and school is at his mams). That's so sad. His dad tries so hard to be a good parent.
You CANNOT think of it this way - hopefully his dad doesn't either. It isn't about your partner - it's about his son's needs.
Is it really so hard to understand? Just imagine for a moment if you were legally required to live between two houses. Every week, a move. Every week, packing your overnight bag back and fro. Feeling disjointed and unsettled because you just don't like it, you just want to sleep in the same freaking bed every night for a change - and then guilty too because you're automatically saying by not liking it, it must mean you don't want to be with the parent whose home is less 'central', when that's not it at all.
That's huge stuff for a 9 year old to process. He's done so well to be able to articulate this to his dad.
And the thing about there still being a 'main' home. Well yes, that's another really quite cruelly hard thing about 50-50 because - there is always, mentally, going to be a 'home' which is a single place because that's how humans think. For it to be truly 50-50, you are actually asking a child to have no fixed home, no one place - because both need to be equal. Mentally, how hard is that?! - For example, think about when you last went on holiday somewhere lovely. You loved being away. Fab time. Then you come home and you flop down and, ahh, HOME.
You, his dad, his mum, all of you have that one home location and you're asking him to not have that, mentally.
If your partner is smart enough to listen to his son and work to give him what he needs, then that will probably cement their closeness about a million times more than pulling a sad face about 'not wanting to see dad as much' and making him keep bouncing back and fro, back and fro.