Hi L., things will get easier, I promise. Jim has been going through ALOT of BIG changes in his little world. Starting Kindergarden is rough on every child. He is acting up because it is a new enviroment to him with SEVERAL new rules. My one BIG advice is have a meeting with his teacher on a weekly basis.In my daughter's classroom they use what they call a Color Change board, and when they misbehave the kids have to change to a different color ( they start on green: Great, then they change to Yellow:1/2 of recess is lost, then they change to Red:all of recess is lost and a conference is made between teacher and parent, I started doing that at home and it worked great, I just changed the punishment's to fit at home ( ex:loss of snack, no movie, loss of games,etc.)I ,meet with my kids teacher's every friday to see how they are doing, and between the teacher and me we have worked together to come up with what works. Now don't get me wrong it didn't happen over night, but they have improved BIG TIME since I have been doing this. Like I said see what the teacher has in her classroom for punishment and change it to fit homelife and start doing it at home. Just try one thing at a time though and do it for awhile so if it doesn't work you can rule that out and try something else.
And yes the new baby is playing on this too. I mean Jim now has to "share" his mommy with something that isn't even here yet. You just need to make sure that you set time aside for you to spend with just JIM. I know that you are tired and I am sure you are ready for the baby to just get here, but even if you curl up in bed next to Jim and read a small book to him at bedtime, it's still special time to him. And when the new baby arrives, let him help you out as much as possible, even if it's "go get mommy the babypowder". The more he is included with the baby to do "special things", and you make it sound like he can only do it, the better he will be. I wish you all the luck and like I said I promise it will get better and easier (my girls are 8 and 6)and my 8 year old was always there to help mommy out with the baby ( she got bored really fast but she still helped and that was good enough for her)keep me posted on how it goes. Good luck and Congrats!!