Really don’t know what the problem is..

Can someone please tell me a reason as to maybe why it takes two-three hours to get my son to fall asleep? This has been a nonstop thing since he’s been a year old. He’s now 17 months. Like this really has to stop... I know kids who go to sleep on their own that are waaay younger than him. I’ve set a routine so he takes a nap at the same time everyday and goes to bed at the same time every night. We don’t do baths right before bed because I’ve come to find out bath time wakes him up making it even harder to get him to sleep at night. I’ve tried melatonin as prescribed by a nurse practitioner... did that for a few days and it seemed to help a little but then after that third day he started waking up at 2 in the morning and not going back to sleep until 5am, so because of that I stopped with the melatonin. I know he’s currently teething so I’ve been giving him Tylenol every other night before bed (I don’t want to give it to him every night unless he’s fevered). We co sleep and I’ve tried the cry it out method as well.