I think it's impossible to know how you, your dp or ds1 will react before the baby comes along. If your son is already 27 months, what have you told him about what's going to happen? He's probably old enough to get used to the idea of having to wait to get what he wants sometimes, spending more time with his dad, assuming he gets some paternity leave/will be helping out more, to get him to think about some activities he'd like to do sitting near you but not necessarily WITH you, eg drawing, sticking collages together, lego, train set etc. so you can have a chance to feed/burp everything else your new arrival.
There's a book my cousin gave me just before my ds2 was born called There's a house inside my Mummy. As it turned out, my ds1 was amazing from the start, which was a huge relief. But I'm sure there are lots of other books to help the penny to drop with older children.
My ds2 was a monster milkboy so we spent a lot of time chairbound reading to ds1, so make sure you have some great books that will endure. And the thing that surprised me most of all was that, in fact, it was usually ds1 I tended to first if there was competition as he (exactly 2 when ds2 arrived) was capable of articulating his needs and creating a truly guilt-inducing scene if he was ignored. I was much less reluctant to let ds2 cry for 5-10 minutes than I was the first time around.
And 2 last things to think about, perhaps: firstly, if your ds is still in nappies, don't be tempted to rush it. He'll see how much attention your new-born gets at nappy changes (and how often it happens as well - I'd forgotten that part) and won't be minded to lose that opportunity for himself. And secondly, if he still has an afternoon nap, I wouldn't be in any rush to get him to stop as that time will be your only quiet time in the day, even if you're just sitting feeding no2 in a chair.
So don't dread it too much as you really can't tell what it will be like. The more support you can get in the early days with things like cooking/washing/sleeping/bathing the better, and, if your ds1 is anything like mine, I'd think through how much to start telling him in advance about what life coul be like.
Hope your last few weeks of pregnancy aren't too exhausting. Very best of luck.