If he moved here he’d see his son every weekend and half the holidays so it’s not like I’m taking his child away from him.
This is you deciding it would be ok for him and his son.
But currently he has him 3 days. He’s getting to an age when friends are so important. Weekend hobbies. Parties. Sports teams. He has already said he doesn’t want to come to the mainland every weekend.
Your DP is prioritising his son. Which is good.
It’s a shame he didn’t think all this through before you rented a bigger house but that works for the times he could come over? And doubtless the provision of the new flat made it possible for him to remain and be a more present Dad.
And if you can’t cope with a long distance relationship, how do you think his 10 yo feels?
It isn’t your Dc being left further away or having their contact time worked around adult needs.
Did you rush or pressurise him, even unwittingly, in your enthusiasm?
It doesn’t necessarily mean he isn’t committed to you as in doesn’t love you, it means he isn’t at a stage when he can live full time with you.
And in reality until your respective youngests go to Uni this may continue.
Only you can decide what your own priority is.