First of all, BRAVO for nursing your son to 15 months!! That is an accomplishment and you are a wonderful mama for making that health and bonding commitment to him :)
I am guessing that because you have nursed him for so long you feel that close bond and commitment to him and you wouldn't want to hurt him in any way. So I truly believe the only way to wean your son is to do it GENTLY! LLL has many wonderful books on weaning naturally and tandem nursing if you are at all interested in that. I have been practicing "don't offer, don't refuse" for several months now. And it has worked quite well. My dd is 18 months and only nurses once or sometimes twice (in the morning/before bed). Occasionally she asks during the day and I just say lets get some juice, or lets read a book, or play something, etc. just to distract her. That always works for us but I would NEVER deny her if she really wanted to, you know.
So I would suggest having Daddy try to put him down at night instead of you for a while. Maybe offer him some milk, water, juice in a cup instead of nursing at night. There are lots of GENTLE ways that you can distract him and work on weaning. But remember that it is a slow process. You don't want to rush anything and risk loosing that close bond and trust you have with your son.
BTW, I would encourage you to do your research on nursing while pregnant, there is not reason you have to stop at any time.
Good luck mama!
And congrats on your new little one on the way!!