My son is 7 1/2 now. He didn't want anything to do with going to the bathroom until he was 3 1/2. (At the time, my daughter was about 10 months old.) He was 6 weeks shy of his 4th birthday when he decided that going in the toilet was better than his pants. It was difficult to remain happy and smiling, but this is what I did: I had him help me swirl his pants in the toilet when he pooped in them. It seems that all my cleaning up after him made it just as if he were wearing diapers that leaked. He had absolutely no motivation to change what he was doing. So, poop on the potty was way better than swirling poopy pants in the potty.
I think I remember reading that girls are "potty trained" at 2 1/2 to 3 years. Boys are slower and are "potty trained" at 3 to 3 1/2 years. And those are averages. Why do some baby books put potty training in the first year? Who knows. One silver lining is that because he was older when he chose to use the potty, he didn't have any accidents. Now, at 7 1/2 he has had an accident or two lately because he doesn't want to stop what he is doing to go to he bathroom. Then, he cleans himself and the pants and puts the dirty things in the laundry room.
Also, keeping you on potty duty makes sure that he gets some attention from you. Children aren't fussy about whether they get positive or negative attention - just that they get some. When you are doing this with your son, then you can't pay attention to the twins. My husband and I took turns with our son in the bathroom. Also, much to my husband's protest, I turned potty training of our son over to my husband. After all, how could I possibly show him how to stand and pee? My husband wasn't sure he could show our son how to do this. But then, I pointed out that he watched as our son was born. If I can push out a baby while all those people watched then he could pee a little. It relieved some of the pressure from me.
The bottom line is that you and your husband are in the best position to know what is best for your son. It won't be long before your son is willingly going to the potty and the twins are sleeping through the night. Now, there is something to look forward to! Good luck! Hope this helps.