Okay I didn't read all the responses but.. in addition to the others....
one IMPORTANT factor is as you said... you are having a 3rd baby in January! That is right around the corner, his Mommy is pregnant and probably busy with other things... can you imagine being your son, and having to deal with all of this?
NO wonder he is whiny! It is that age also...but my gosh, you really ought to PREP him fully for the new baby being in his life... for a child, this throws them for a loop. And be prepared that he may get even MORE unhappy and whiny and unable to COPE with the changes in his life.
PLEASE keep in mind, that his behavior does not warrant a "bad boy" punishment. Right now, to me, he needs TOTAL understanding... he is going to be an "Eldest" child... and by default, adults always usually expect "eldest" children to be "perfect", more mature, more obedient, more 'smart', to be the 'example' for ALL the other siblings, to be EVERYTHING to everyone, to be SO MANY THINGS THAT IT CAN STRESS THEM OUT and cause emotional hardship on them, when they are simply not "ready" or old enough to handle it all by themselves. Poor thing. This is so much to expect from such a young child.
Keep in mind his age... this is a lot for a 4.5 year old to be expected to handle. And like anything else... a child this young needs to "transition" to things... some children take a LONG time to even adjust to having a new sibling AND THE CHANGES THEIR MOMMY will have in relation to them.
Keep in mind, that the main "symptom" for a child who is experiencing "STRESS" is: BEHAVIOR REGRESSION. THIS is what your son is having. Spot on.
So, the solution is not to make it worse for him... he needs hope, he needs coping skills, he needs understanding, he needs a soft place to fall, he is probably feeling insecure too, he needs LOTS of attention and comforting, he needs to know he can TRUST the situation, he needs to know he can RELY on his parents, he needs to know that he is LOVED no matter what, he needs to know that his "place" in the family will not get affected when the baby comes and even AFTER... he needs a lot right now.
He is the "eldest" and by default... the eldest child ALWAYS bears the brunt of having to "shoulder" the world on their shoulders... this is too big of an expectation for a child, and too big of a burden to carry for any child.
No, I would NOT ignore him.... not at all.
There is a difference between "defiance" and a child needing what they are lacking.
My daughter, she is 6 years old....she "whines" too...but it is always when she needs "me" more... and she will even say it and call me on it. Point blank. I never scold her for it, but I ALWAYS encourage her to speak out and TELL me whatever she is feeling... I rather have a child that can express herself, than one that is always "pent up" and frustrated. That is worse.
ANY child, and ESPECIALLY boys, NEED to be "taught" that they can express their feelings, and to feel 'safe' doing so. ALL kids "whine" no matter what age, even teenagers whine. So it will not "end" with finality. But simply teach the child "how" to express themselves more effectively. For me, I'd much rather have a child that expresses their feelings... than one that is taught they can't or that it just falls on deaf ears. A child who is taught their feelings and words don't "count" will have less morale....and it kills their spirit and joy. Also if your son whines when he doesn't get "his way" then he needs to be taught about cause and effect.
I recommend the book "Your 4 Year Old" or "Your Five Year Old"... you can find it on www.amazon.com It really explains the behavior of each age "phase." It's a fast easy read.
Be thankful he is a good student and "behaves" in school. That is great. But at home, yes, most kids are just at their wits ends too. And they let it all hang out.
All the best,
Susan