I think you and your DP like an easy life. You find it easier to blame the 'wicked ex' than to do anything about his relationship with his son. I'm happy to accept that she is controlling, that she did what she could to prevent the relationship when he still lived nearby with his parents.
But...in that situation, you and your DP decided to move in together, 150 miles away from his son, living a lifestyle that prevents him from visiting more than a few times a year. Not going to court and making a commitment to a proper contact pattern, building up to overnights and holidays in a concrete, child-centred way that the child and his mum could rely on, but phoning and visiting when you get round to it and bitching about how terrible ex is when she won't cancel everything when daddy decides he will turn up.
And then into this volatile situation, in which you aren't willing to actually do anything about improving the contact, you decide that the very best thing to do would not be to move nearer, or make financial decisions that would mean he could visit more often, or go to court, no, the very best thing would be to insist that the miserable amount of contact time be 'shared' with you.
Thus nicely giving you another excuse to bitch about the ex, and turning the spotlight firmly back on you and how awful the whole thing is for you. Not the child, or your ex, who are 'perfectly happy' with only seeing each other a few times a year
. So don't go, why do anything difficult in order to increase contact? You obviously don't care what anyone else in the world thinks about it - much easier to start hurling abuse and saying we are all bitter and twisted than do any difficult thinking about what is being said.