I haven't read the whole thread so I'm not sure what everyone has said, but here's some of my suggestions (and by no means am I any kind of exert, my kids are both terrible settlers and sleepers too!

*Maybe try putting him to bed earlier. Just by 5-10mins every 2-3days or so, but maybe a 7pm bed time would be better for him. If he's not having day sleeps and not settling for an hour or two, at least if he's going to bed at 7pm, then he'll be asleep by 9pm at the latest, and he'll get a good 10hrs sleep. I think they still need at least 10-12 hrs sleep a day at his age. And as the saying goes, sleep breeds sleep.

*Maybe try giving him things that will help him sleep, like a small glass of milk before bed, rub some scented oils/creams onto his skin after a bath (massage and relaxing scented oils always make me sleep!) maybe some rescue remedy into a glass of water??

*Have you tried just asking why he doesn't like to go to bed anymore? Is bubs still up when he goes to bed? If baby is still up maybe try putting them to sleep at a similar time with bubs going down first (I know, not that easy!! But if possible it might work!) Maybe let him help set a routine, ask him what he would like to do when it's bed time (ie how he wants to relax, with a story/massage/cuddles with mum or dad on the lounge, if he wants a night light on in his room to fall asleep with, if he wants you to lay with him to fall asleep.....)

Good luck. I hoep you can work it out. I know how frustrating sleep issues are. If it helps you feel better, DH and I are still co sleeping with both our kids, dummies, blankies, laying with them for up to an hour.... so very, really, really REALLY not ideal or how we want it, but it's what we can deal with at the moment, so that's how it is!

Above all consistency is paramount. I know it's hard but if both you and DH can be on the same page doing the same routines, then it will be easier for him to get back into a routine.