How old is he exactly? Closer to 2 or 3?
I wonder if he could well be OT at bedtime and that's part of why he struggles to settle, that's quite a long day on a fairly short nap. However if he is low sleep needs or closer to 3yo it may well be some 1-0 issues are adding to your troubles and he's actually needing a consistently shorter nap. Do you notice that he settles quicker on days where he has a longer or shorter nap? Does he ever skip his nap completely? Have you ever tried an earlier bedtime?
Ther are also other things going on for him right now, the arrival of a sibling is MASSIVE for toddlers and can disturb the sleep of even the most angelic of sleepers. Add to that molars and disturbed sleep is somewhat inevitable. However I do think you could start to work on a gradual withdrawal plan to get him sleeping in his own bed and stop co-sleeping if that's what you all want.
Having re-read my BWSAYP book, I'd suggest something like:
The day you decide to start, talk to him about how tonight he will be sleeping in his own bed on his own. (Keep the dummy definitely for now - that's a comfort item and I wouldn't be getting rid of it at this age). But reassure him that Daddy will be right there in the room with him. Get Daddy to put a blow-up bed/mattress on the floor next to Reuben's bed. (I say Daddy because he is going to need to be there all night to begin with and you can't do that if you have a newborn to feed).
At bedtime, do your routine as usual and have cuddles on the bed. You can give him one of your items of clothing with sleeves to snuggle if that's what he likes. But explain that he will be sleeping on his bed, and Daddy will be on the mattress. Daddy needs to go to bed at the same time as Reuben - Tracy's suggestion was to take a torch and book in with him so he can read once Reuben is asleep if he's not ready to sleep himself at that point.
Daddy should tuck Reuben in, get into his own bed and then lights out.
Reuben will almost inevitably have a massive tantrum, and try and get in with Daddy. Daddy needs to calmly but firmly return Reuben to his own bed, and then get back on his mattress. Repeat repeat repeat until Reuben goes to sleep. This may take hours - and Daddy needs to be prepared for that.
If Reuben wakes at night, repeat repeat repeat.
The first night Daddy should expect not to get a lot of rest but staying consistent will be absolutely key.
I would do exactly the same for nights two and three.
Night four Daddy could get up and leave once Reuben goes to sleep, but should return and stay if he wakes in the night.
Hopefully he will be getting the message by now, and Daddy will have to go in less over the next few nights.
The next step will be taking Daddy's mattress out of the room. In the BWSAYP book Tracy does this by talking to the child in question during dinner time about how tonight we will do the same routine as always, but Daddy will set a timer (for 3 mins) and when the timer goes off its the end of cuddles and Daddy will leave.
You would expect Reuben to protest a lot when Daddy leaves while he is awake, but Daddy should go back in and return Reuben to bed before leaving again. Repeat repeat repeat until he goes to sleep. Again this may take a very long time the first night, but you should see huge improvement in a few nights if you are consistent.
What do you think?