S.H.
Great suggestions here.
Yes, perhaps adjust the time he takes his nap....
Also, you said he's been waking up "screaming" in the middle of the night... this could be because he is having night mares or "night terrors." These things are purely developmental, normal, and begins at about this age.
If you look it up online and do Google searches on the topic, you will find lots of information.
You also mention you are pregnant and tired. By this age, when I was pregnant with my 2nd baby... I would literally tell my daughter that "Mommy has to rest... and take care of baby...." and I would enlist her "help" and literally lie down on the sofa, put her toys next to me, and she would "understand" and "let" me rest... she would even rub my tummy (her brother) to help me. It was also a great "bonding" time for us together, and she felt close to her baby brother this way.
Your son is also having LOTS of changes in his life now, because you are pregnant. It can create anxiety in the child and perhaps upset his usual sleeping pattern. This is normal. But, for us, we completely talked with my daughter about it and included her in everything as much as possible.... I even took her to my OB/BYN appointments and my Doctor would let her "help" by teaching her how to put the heart monitor on my belly during check ups. We also talked with her about how she will become a "sibling" and etc. ALL of this helped make her feel less "insecure" about the whole thing. It was fun for her, and it made her feel loved and "important." And by the time the baby came, she was completely secure and happy about it, with no anxiety....and she adjusted so great.
This age is an age of great changes for them anyway. So, on top of just his normal developmental things, this is just maybe "other" things that maybe are affecting him.
Just some other ideas,
all the best,
Susan