Dad doesn't see son very much

[Slowway]

My ex partner has recently come back into our son's life after a 6 year absence. Our son is 10 and was desperate for me to make contact with his dad. I did so reluctantly, as his dad was a very hostile and difficult person in the past. He also hasn't made any attempt to make contact with us in that time.
I did some research on him before reaching out, he has paid maintenance consistently for a year (first time since we separated), and is now medicated for his ADHD. He had previously refused this when we were together.
We met up before and discussed how we'd like to move forward, helping him and our son build a relationship. We agreed on two hours every Sunday, to which this quickly became very other Sunday. I respected he had other commitments with his partner and according to him, long and unpredictable hours in work. The agreement was time would increase as the bond was built. Fast forward over six months and he has made every excuse not to increase time with him.
My son is old enough to understand that mum is asking the questions, but dad is saying no. He has asked if he could start having sleep overs in his dads, to which he said yes to our son but then made it clear he couldn't have him overnight because of work apparently. Whenever we've agreed to increase his time, there's always a sudden emergency that comes up and he can't have him more than 2 hours. It's always very last minute too, something that is not a new trait. He was very controlling in the past and part of me feels this is still an aspect.
There's times I've reached out in order to confirm details but he doesn't respond. I can't make any plans myself and this is really the only time I have free all month, except for working 42 hours a week. I've tried to accommodate him by changing weekends and being ok when he's cancelled last minute, but I'm confident he wouldn't do the same for me. My son is now asking all the time why I can't make him make more effort and I really don't know what to say or do.
Has anyone been in a similar position and could offer me any meaningful advice?

That's so desperately sad. It's awful for a child when their parent just doesn't care, and sadly it seems that your ex really just doesn't care for your son.

All you can really do, IMO, is continue to try with your ex, ensuring your son knows you're trying.

As sad as it is, your boy will wise up and eventually get sick of attempting to get his father to be the father he wants him to be, and give up.

Im so sorry for you and your poor boy. I think the only thing you can do is keep trying to facilitate the contact and keeping your son informed that you're doing this. As PP said, eventually your son will realise what a POS your ex is, but this will be really hard for him to accept -you'll be there to reassure him that none of this is his fault, there's nothing wrong with him that means his dad doesn't want to see him.

If he goes 2 hours to see your son and he can't do more just keep.it that way and it's not the time he's with him in the quality time. If he needs to.work and that's the excuse ..he's telling you that he can't so tell.your son . That dad is working a lot and can't come for so.many hours but at least he comes to.see.him never talk bad about the dad to your son ..also he has a condition that is difficult to deal.with and medicated ...

@DearAbi this man is actively choosing NOT TO! Not that he can't, he just can't be arsed. No hard working mother should be expected to make a excuses for a dreadful excuse of a dad. Certainly shouldn't have to be covering a crap dads tracks!! I have autism but I still ensure I'm a good parent to my child. This child is old enough to see for himself and doesn't deserve to be lied to and made to doubt his own understanding. A shit dad is a shit dad. No mum should have to cover for that.