Yes OP,I've been where you are now and honestly my 2 oldest DS's 20 and 18 aren't bothered about they're Father,they call my DH Dad because he is they're Dad.
He's the one who helped look after them when they've been ill and he still does even though they look like and are the size of older men,he's the one who's done the school runs,been to they're parent evenings,helped sort out any problems at school or with friends,he's the one who's celebrated they're birthdays,Easter,Halloween and Christmas with them,listened to them read,helped with homework,bath times and bed times,read to them,played games with them,cooked for them,picked up after them,got them up for school and work,gave them lifts out with they're mates,gave them lifts to work,picked them up when they've been a bit worse for wear of a weekend,he's the man that they laugh with,confide in and moan at.
He's they're Dad,not my ex husband,he never really did anything with our sons when we were together and once we split up he just got worse to be honest.
They're Father now regrets it and try's to have contact with them and we try to encourage them to but it's hit and miss if they can be bothered with him but I did warn him when they were growing up that this day could come,in the end honestly the one that will miss out the most is your ex OP and if he's unlucky enough by the time he's realized that it could be to late for him to do anything about it!