Is there some way to perhaps put your son in daycare/preschool? At this age, they do get more active and need other kinds of stimuli. Perhaps for your son, it will tire him out... and they do have naps at this age... and maybe it will get him more regular?
Also, is it possible to separate your son and daughter? Being in the same room is creating sleep problems for him... and disturbances for your girl... and you/hubby having to run interference so he does not wake his sister. At this age...it will probably continue, until he matures enough to where he can "not" wake his sister. For us, we have to separate our kids...otherwise they do NOT sleep, or they wake up the other.
I remember my daughter at this age.. she too went through a "phase" like this... she would not want to sleep, wanted to stay up, wanted to keep playing, wanted our "company" while she fell asleep, on and on. Yah, it was tiring and took a lot of patience. But she grew out of it. It took encouragement from us, talking to her, incentives etc. But in the end... all the strict or flexible methods we used didn't work. She is just so cerebrally active that it takes her time to "quiet" her mind, and then fall asleep. My son on the other hand can sleep no problem.
At other times, we would just tell her that WE are going to bed. We'd get ready, turn off the lights, and hop into bed. Yes, she was left there, "surprised" that we actually went to bed... and then she would go to bed "too." Sure, we kept our eyes and ears open and watched her & how she reacted...but we tried the reverse psychology thing. And, at times, it DID make her go to bed. Because then the entire house was dark and no one was awake to "play" and no other choice for her.
Also at this age, they are changing so much, cognitively, and it throws off their routine/ability to sleep. Just KEEP to your bed routine/times/schedule... and keep it consistent. He will return to regularity. But it may take time. It's a bump in the road now. But keep enforcing it.
At other times, at this age, my girl would ask if she could just sleep on the floor... this was a phase she went through. We let her. It made her sleep. She'd get blankets and pillows and make "her spot" all comfy and then she'd settle in, and start to drift off and fall asleep. We let her do this, and sooner or later, she passed this phase and then went back to her bed.
Also, kids need a "wind-down" time before sleep. I have quiet time pre-sleep... so that they are not all keyed up when they do hit the sack. I even turn off the lights except for one, and make the room dim... then get them ready for bed.
Another thing is homeopathic sleep aids for kids. My friend used the "Hyland's Calms Forte" when they go on trips. She said it's great. There are no contraindications on it. You can probably get it at any natural food store or Amazon.
I know it's not easy, and your and Hubby's schedule is not easy either... it is probably a phase... but who knows how long it will last. Each child is different. For our girl, at that age... well, it was a 2 year old thing. She was not naughty or obstinate about it... she just could not go to sleep and she was changing so much. We never punished her for it.. but we tried our best to keep to our bed routines. Eventually she got better at it.
All the best,
Susan