*Adding this: I remember when I was that age (and older), I used to, in the middle of the night, get lonely and scared in my room by myself... and I would get up, walk down our dark scary hallway, and go to my Parent's room and squeeze myself in between them. They let me. I grew out of it in time. I went off to high school and college just fine and was perfectly fine. No harm.
Kids in addition to "separation anxiety" also evolve in terms of their "bonding" with their Mommy... it changes and fluxes and sometimes they DO get more clingy or needy or missing you even if you are right there in the same room. Its normal though.
They go through various stages of separation anxiety at each age-set.... and its manifested in different ways, again per their age.
And yes, at this age (and onward) they get "night mares" and night time 'fears' and feel alone.
My son, from about 2.5 years old, which he is now, has "changed" and he's gotten more "clingy" with me in a very cozy way and he is always telling me "my Mommy..." and "Love Mommy..." and then he just wants to be close to me and hangs onto me like a baby Koala and just smiles like he is so happy just being on my lap. I know it's just a phase.
But, I also co-sleep with him, when need be.
We have a floor futon on the floor of our bedroom, where our kids can sleep or co-sleep when need be. We don't mind.
My daughter is 6 years old, and she STILL gets occasional 'fears' at night.. .and gets genuinely 'scared' at night. Its just her imagination, but which is normal stuff for this age. We just console her and my Hubby even checks the outside with a flashlight... but she still gets scared of any little night time noises. We let her sleep in the floor futon in our room whenever she wants.
Your son is normal. It's "growing pains" too, of which there are many things a kid goes through.
You are also pregnant, and so naturally, this affects a child. He probably "misses" you, and knows darn well that another baby is coming. Just keep him in the loop and they need extra attention and reassurances. When the baby does arrive, and gets home, he will need extra of everything... and he will need lots of time to adjust... and its normal.
When I was pregnant with my 2nd child, I involved my first-born in everything... we took photos together each month with my growing tummy, I took her to my pre-natal exams and the Doctor even taught her how to use the heart-beat monitor on my tummy, we went shopping together for baby and I let her pick things herself as a "gift" for her baby brother, I let her massage my tummy and just let her talk to me about it and ask questions, I asked her to tell me anything she was wondering about, about her baby brother etc.
Once her brother came home, I spent every extra time I had, with her. COnsistently. My Husband did too. We kept her own routines the SAME.... with the thought that she should not have to give up anything, just because she was now the "oldest" & another baby was home. We kept everything consistent for her and "special" just for her. A child needs that.
It will be busier with 2 kids... but more time has to be spent on the "eldest" child too. Or they will feel lost and frustrated and irked. LET him adjust to it all at his own pace.. and let him express any feelings/frustrations to you, and let him know it's OKAY to do so. Once they know they are still "important" to you, it helps them. And talk with him about it. I would ask my daughter "Tell Mommy anytime you are feeling frustrated... or something is bothering you now that you have a brother..." She felt validated that way and it helped.
Engage your son to "help Mommy" and show him all about his new baby sibling. Even when you have to rest... tell him "Mommy has to rest now... can you lay next to me?"
For me, when I had my 2nd child, it was also when we enrolled my first-born into Preschool. She LOVED it and it was HER own "special" thing, once her baby brother came home... and it gave her a break too, to just have time with her own friends and routines & it was a positive outlet for her. And it gave ME time to just be with my son as a baby, and get us into a routine too.
Your son, like most kids, has many things going on at the SAME time... so, even at night. And well, its not easy for them either. And his life is changing... his Mommy is pregnant and he is going through his own developmental changes as well.
A good book is "Your 3 Year Old" or, "Your 4 Year Old" which you can get from www.amazon.com It's a great series that explains EACH age-set, so we parents understand what each age goes through and it's characteristics.
Good luck, sorry for rambling,
Susan