Sadly you can't force love.
I spent too many years bending over backwards to facilitate dds relationship with my ex and even then he frequently let her down.
It got to a point I could no longer afford to do this and dd got wise to the dynamic and actually asked me to stop - she wanted to know he wanted to see her, cared about her. She also asked me to stop covering for him - which I had been doing thinking it was better for her. She said that actually it made it harder for her express her anger and frustration and hurt at his behaviour, that he didn't deserve protecting and we should both be honest about him from that point on.
Very quickly it went from him seeing her 5-6 times a year due to my organisation and expense (it was costing me more than even what he should have been paying in cm which he didn't pay consistently) to once a year for a couple of years and at this point (dd now 18) he hasn't seen her for severely years purely because he Cba making even his own arrangements and she's lucky if she gets a generic birthday card - late!
It is utterly heartbreaking to see her feeling as if she is lacking in some way because her own bloody father makes sod all effort to have a relationship with her.
We do have a close relationship though and certainly in the last couple of years we've had conversations where she has said that she really appreciates all I've done and how hard it was for me.
One of her friends is currently going through her parents breaking up and the dad is actually stepping up. The friend has at times expressed frustration and disappointment if he's had to cancel plans due to work (he works shifts/zero hours contract) and dd does point out to her that actually he's a pretty good dad and sees them whenever he can, pays maintenance and buys them things they need in consultation with mum.
Dds dad has recently been in touch trying to re-establish contact but it turned out to be he wanted a favour of dd and that was why. This was so hurtful to her. In addition he didn't even get his backside organised anyway for dd to go to theirs and the plans fell through. I'd warned dd not to book travel plans until she heard for definite her dad had organised his end and she was glad she took this advice. He'd have left her seriously out of pocket and not given a shit!
Op unfortunately all you can do is support your DC, don't do him any unnecessary favours and accept you can't rely on him. I wish I had done the latter far earlier than I did.
It's not fair, it's not right but as pp are saying you reap what you sow.
You will have a lovely close, appreciative relationship with your DC, you'll have great memories of the things you've done together (even the things that at the time are done eg out of financial necessity kids can see in a very different light. My dd still fondly remembers an "indoor campfire picnic" we had once when I was too skint to top up the meter and was trying to make the best of a bad situation).
He will not have that closeness, they won't confide in him, they won't have warm memories of daft events and when he's older and wants to reconnect they won't be keen - I've seen this with friends my own age and slightly younger who had absent dads who suddenly when they neared retirement age/2nd wife left and they were lonely tried to get their now grown kids to fulfil THEIR needs. Those adult DC very much think "he Cba with me when I needed him why should I go out my way just cos it suits him now?"
I wish I'd let him gradually fade out of dds life when she was much younger it would have been much easier for her. I think the advice often given to do all you can to facilitate a relationship with your child's father following a split even if they're showing clear signs of not being interested is wrong. If it's obvious they're not interested instead give them enough rope... They'll either step up or they'll disappear while the DC are relatively young.