My first 2 kids are 16 months apart. As a first time mom, I made a lot of "mistakes" with my first born through trial and error as we all do, and one was not teaching him to sleep well at night. I, like you, was worried when the second child came, what I was going to do!!
First of all, you didn't say how your son GOES to sleep at night. Do you rock him to sleep or lay with him or offer him any other type of "crutch" involving you or your husband being there with him when he falls asleep? If so, no wonder he wakes up crying. If you fell asleep in one place and woke in another, wouldn't you freak out? Or if you fell asleep with someone there and you wake up and they are gone, that would understandably freak out anyone. So you need to start with a routine to get him to go to sleep on his own. That way, when he wakes in the middle of the night (like we all do, even adults), he will know that he can fall asleep on his own and just roll over and go back to sleep (like we all do.)
***By the way, if he is already falling asleep on his own, I will throw my hands up because I have no idea why else he would wake up crying hysterically in the middle of the night***
OK... here is what you need to do... Establish a bedtime routine. It can be ANYTHING as long as it is the same every night. Many people read to their kids at night, for example. I actually have never found night time reading to be enjoyable or comfortable, so it's not what my family does. For my preschooler, when I let him know it's time for bed, we begin his little evening rituals... I brush his teeth, then he goes around the house to give everyone else a hug (he has 2 older siblings), then he gets a sip of water, then he goes potty, then he climbs into bed and I sing a version of itsy-bitsy spider that we made up toegther that has 5 verses... THEN (and this is the part where you come in---- do whatever YOUR routine will be with your son... then do this...) I tell him I will be right back and I LEAVE THE ROOM. If your child isn't used to falling asleep on his own, he will cry-- possibly hysterically, That's OK. You are NOT, NOT, NOT going to abandon him and allow him to cry it out. You are not even going to be gone long enough for him to get so upset that he throws up. You leave the room and literally you can stand outside the door and count to 10 slowly, then go back in. You let him know that you said you would be right back so you came back. Calm him down. Try not to pick him up if you can calm him by patting him or singing to him or anything else, but if you MUST pick him up, the important thing is NOT to leave the room holding him, and NOT to allow him to fall asleep while you are holding him! Leave the room again while he is still awake and tell him you will be right back-- you can tell him you are going to do something boring like change into your PJs and be right back, or wash the dishes, or do some laundry or whatever. You don;t have to actually do those things-- but the idea is you are letting him know that there is something else you need to go do real quick but you WILL be back. Then be good on your promise and go back! Just keep making it longer and longer in between how long you leave him and when you go back in to check on him. For my son, he is 3.5 and I have been doing this for a really long time, so I usually only go back in once after 5 minutes or so. Sometimes he is already asleep. If he is still awake, sometimes he wants me to come "right back" again, but I usually just tell him I'm going to sleep, too, and I'm not coming back again tonight, so I make sure he has his covers on and he is cozy before I go. But for your son, you may need to go in and check on him over and over and over and over and over and over again for the first 3 nights or so. By the end of a full week of doing this, if you were consistant (NO taking him back to your bed, NO taking him out of his room, NO laying with him until he falls asleep)he will be sleeping MUCH better throughout the night.
No one had told me this method with my first born. I thought I needed to either let him cry it out or go in and pick him up and take him back to my bed. I had no idea what else to do. Well, here is your best method. I used it for my 3rd child and it changed my life!! You don't have to feel bad about him crying because you don't let him cry for very long, you go in and assure him that you have not abandoned him, you calm him down, but you do NOT allow him to use you as a crutch for falling asleep.
Good luck! You will be a sleepless zombie once the newbon comes like all new moms are, so nip this NOW! Start tomorrow!