You sound like an amazing mom. Congrats on your healthy 1yo and your second on the way. I'm sure that you can handle either scenario (tandem nursing or solo), but I understand your reservations. I had similar reservations, and chose to only nurse one child at a time. My first and second child were 23 mo apart, and my 2nd and 3rd were 18 mo apart. Weaning my first child was the most challenging, since I similarly had expected to wean by her first birthday. She really was not ready by her first birthday, and would use her sign language for milk frequently. By 14 mo though, she had arrived where it sounds like your baby is (only nursing to go to sleep or for needing extreme comfort). By then, I was pregnant with my second, and I really was ready to wean. BTW, I had stopped offering the breast by this point, only responding to her requests. At 14+ mo, I started watching Baby Einstein videos with her during the typical nursing times with her snuggled on my lap, hoping that it would distract her sufficiently and provide the closeness she needed. She really enjoyed them, and I would point out colors, shapes, and animals, which she really responded to. I was so tired with my pregnancy, that I welcomed the down-time with her and the snuggling. Anyway, by the time she was 15 mo old, when she asked to nurse (using her signs), I tried to distract her. If she asked again, I would nurse her. During her 15th mo, she went 3 days being easily distract-able or not asking, so I decided to hold firm and declare our nursing relationship complete. When she asked on the fourth day, I told her that she was getting so big that we didn't do that anymore, and offered her the sippy cup, which she was used to getting water from. She was a little sad, but was receptive to taking cow's milk from a sippy. (She had been taking cow's milk for a month or so from a sippy, so it wasn't a total shock).
I know the age difference between our children is different, but I hope this helps. This process, although significant, was not traumatic for either one of us, but it did take a while. If you hope to have your son weaned before your baby arrived, start now and hopefully you can do it gradually enough that it will feel good to both of you. Congrats for nursing for over a year! It is an amazing gift to your son (and to yourself). If you do decide not not tandem nurse, do not offer mixed signal by giving in to requests from your son. It is very typical for your firstborn to ask for what the baby is getting, or want to be on your lap when you are holding the baby. Gently remind him that you don't do that anymore, and if at all possible, offer him some snuggle time.
Good luck! If you think I can provide any additional support, please let me know!!!