ah yes, here you go....! I have a toddler who is 20 mos older than my twins. the twins are just over a year now. it's been a looooong year, but we've made it, and things are indeed, in general, getting easier.
if you're in Montgomery County, you can join MCPOM, the Mont. Co. Parents of Multiples group (http://www.mcpom.org), which I highly recommend. you didn't say if you're having fraternal or identical twins (if you know), but that ~can~ (doesn't have to) make a difference in terms of how high risk your pregnancy is (identicals ~can~ be higher risk, but even if you're having identicals, don't get too caught up in the risk factors, it's not worth it). I'm happy to talk more about that stuff if it's even an issue for you (I carried my mono-di ID boys for 39weeks +1 and delivered naturally after being monitored very closely the whole time, which will make sense to you if it needs to and if it doesn't then don't worry about it... :) ). if you're carrying fraternals, then you're really not at a higher risk than any other pregnancy (absent other risk factors, of course). seriously.
I am happy to talk to you about anything as you head down this road. my biggest piece of advice is to take help where and when it is offered. seriously. start a list now of things that people can do for you, so that when someone offers (and they will, but some will only offer once, so take the offers when and where they come. you need them!) you can point to your list and say "sure, could you mow my lawn? how about groceries, can you pick up some groceries? diapers? do laundry? clean the bathrooms? change kitty litter? change a baby? hang out with the kids while I take my first shower in 4 days? take your pick!"
learn to say "yes, thank you, that would be lovely." take help. don't try to do this alone. it is too much for even Superwoman. trust me on this. figure out the maximum help you can afford and tolerate and go for it. that's different for everyone, but making sure that you carve out time for your first born and time for yourself and time for your spouse are really hard to do, on top of everything else, and the hormones and guilt aren't going to make it easier, so plan now to need more help than you think. you can always pare stuff down. I found that after a couple of weeks I actually asked my mom to leave (nicely, lovingly, gratefully) b/c we were ready to go it without the full-time, live-in assistance of my mother. but shortly thereafter we had to hire someone part-time b/c I still needed a lot of help. she's still with us and I would not have survived this year without her.
as for introducing your son, we waited until I was about 20 weeks to start talking about it, by which time I was already pretty huge (people started the comments about being due any day now at about 22 or 23 weeks.... made me crazy.... I look at the pictures now and think good god, how the hell did I do that?!?! :) ). we let him feel the babies kicking, we talked about them being his brothers (I have 3 boys now), we talked about what a great big brother he was going to be, how they were going to be his special babies, they were going to love him and look up to him, but he was still always going to be mommy's special little guy, too. I dunno, we just talked about them a lot. we named them so that they would seem more real to our older son, I don't know if that helped or not, but it made us feel better. he didn't understand really, but he was very sweet and loving to the belly, and equally so once the babes were here. don't get too caught up in forcing it; when he's interested, talk about them. and when he's not interested, let it go. our kids have been really close, our oldest has always been so gentle and loving with his brothers (not necessarily with us, though; do prepare yourself for him to lash out at you when his whole world comes crashing down.... and it's really really hard to get through, but it only lasts a few weeks.... during that time, you have to do everything in your power to show him that he still matters, that he's still important, and don't prioritize the babies over him, even though that sounds like an impossible task.....).
this is a crazy long post.... if you want to talk about it more, I'm happy to. I exclusively BFed the twins as well (and am still nursing now), so if you want to talk about that I can help. anything you want to talk about, if I can help you I will. so will the moms and dads in a POM group like MCPOM (if you don't live in Mont Co).
it's a scary journey, but also a great one. I wouldn't do this past year over again for any amount of money, but I did survive it, we all did, and you will too. that's important to remember....
good luck!