Hi J.,
I can sympathize with you, because my son has developed some fears and seems to have trouble falling asleep occasionally because of them.
I find that it is very helpful to let him talk about it as much as he wants to, and to be open to the fact that some things can be very scary to a toddler, but to reassure him that you are always there to protect him, and he can feel comfortable telling you.
Also, another routine we have is to read his favourite books before bedtime, and make sure he is tucked in with his favourite stuffed animal.
Finally, I sometimes sing songs and give him back rubs, and then we talk about highlights of the day, and then I tell him things that he can look forward to the next day (e.g. - 'The Sun is sleeping now, so all boys and girls are sleeping, too. Tomorrow when the sun comes up, we are going to go to the library to read some books and then play at the park!'). This has helped him to settle down and look forward to the next day.
While I only have one child so far, I have observed in other families, that the older sibling of a baby-on-the-way tends to need a little extra TLC, perhaps because of the excitement around the new baby, or because the older one misses the feeling of being a baby, and being so warmly and unconditionally cared for. We put so many conditions on things as children get older, it's no wonder so many of them feel like they have to "earn" love, or that they need extra attention. Anyway, that was a random thought, but what I mean to say is that your son probably needs you a lot right now, and having nightmares is one manifestation of this.
Best,
Nessa