If anything, seek the advice of a Lactation Consultant and see what they say. I'm sure they deal with this concern all the time.
Try transitioning him with a bottle?
Some Moms still breastfeed during pregnancy... this is up to you. But, your OB/GYN may recommend otherwise. Did you tell your OB/GYN you are still breastfeeding?
Try researching it.
Once, I was pregnant when my first child was still nursing... very infrequently by this point, but I let her self-wean, and she did "stop" by the time she was about 2.5 years old. But, my OB/GYN, although he was very pro-breastfeeding, told me to stop when I was pregnant. With this pregnancy, I had a miscarriage at 6-weeks. It wasn't because of the breastfeeding though... I don't believe. It was just not developing well. I did get pregnant after that, with my son, and by this time my daughter had stopped nursing. Once my second baby was born, she was fine and didn't regress... and even though I was nursing my 2nd child. BUT, it also had to do with her age... she was more mature by then and old enough to understand.
Since you DO want to wean him...then it's just a matter of doing it. Try the lullaby CD since that worked for you before.
Mostly, with weaning... you just try to replace it with something else (ie: a bottle or sippy cup), or a lovey of some kind, or another activity..... THEN shorten the duration of it.. .don't let him just stay there at the breast indefinitely.
You are lucky he has stopped his night-time nursings, since this is usually the most difficult one to stop. Perhaps also, instead of just putting him at the breast when he wakes up or naps... just try "not" doing it. THEN SEE if he "forgets" about it, or asks for it. Usually, as you wean... (I let my kids self-wean... but also "let" them ask, and did not "automatically" put them at my breast) you can just try going according to HIS "asking" you. Don't just do it automatically.
And then, he may not ask for his morning or nap breastfeeding. That is what I did with my 2 kids... they self-weaned but it ALSO means letting THEM "lead" it, not my deciding "when" and "if" I nursed them.
Since you said that his morning and nap-time nursings are probably more for "you" by now... then just make these nursings less prominent. Only do it if he "asks."
Mainly, it has to be something that you do want to do. And once the baby comes... hopefully he is comfortable and feeling secure about it all, and he will be almost 2 years old when your 2nd baby comes. So prepare him for it... eldest children need to feel "informed" about their sibling... and still in the loop and a part of it all. Having a baby, ALSO means, preparing your eldest child for a baby too. It's big deal to them. You want them to "like" the new baby and their "new" role in the family etc. When I was pregnant with my 2nd child... I mostly spent the time "prepping" my eldest child for the baby... so she felt good about it all... and not so "shocked" then her little brother came home.
Well I'm sure it will be fine. It will all work out. But do what you feel is best.
take care,
Susan