I can relate... I'm a SAHM too, and when my girl was about 3 years old when I was pregnant with my 2nd child. My girl too, was/is very bright for her age, and was active and LOVED having ME play with her... but this is normal for a child.
What I would do is... actively incorporate her into my pregnancy... talked to her about it, took her with me to my OB/GYN appointments (my Doctor encourages it and even taught my girl how to put the heart-beat monitor on my tummy), I explained (in an age appropriate way) that as Mommy's tummy gets bigger, that I would need her "help" so that I can get rest and not strain myself etc. Once a day, I would even take a little "nap" for myself... incorporating my daughter- I made it a routine for us... I would lie down on the sofa, explain that Mommy needs to rest now... and have her stay with me in the room, put toys and her favorite things there with her.... and explain that it is "quiet time" now... and that Mommy needs her to "help" me and "baby" rest... she completely understood and would even be "proud" that she is "helping" Mommy and her baby brother by letting "us" rest. She would even put her hand on my tummy and sing little songs for us and would even get me a pillow to get comfortable and what not. The main thing is that I made it a "routine" for us... and, by emphasizing "team work" and her "role" as a "big sister" and "sharing" my pregnancy with her... she was so proud to participate this way... and to this day, I am still so amazed that she would understand AND even "help" me to rest.
I also explained the things I "couldn't" do as my tummy got bigger... and she stopped asking me to carry her or to lift big things etc. We made activities of taking photos together, with my big tummy...and showed Daddy later. I let her use fingerpaint to "paint" my tummy too, and it was real fun for her. It gave her a nice way to "bond" with her baby brother in my tummy, and gave me a nice respite when I needed it. I also let her make things for her baby brother and "projects" as such... which made it all the more fun for her. This was thereby an early "lesson" in "sharing" and "empathy" and being a "big sister." I also got her a doll and let her "diaper" it and "feed" it etc.
At your son's age... perhaps can a friend come over, or do you have a friend he can stay with sometimes to give yourself a rest???? Being that you are having "complications" with your pregnancy at this stage... It would be really great if you can get another pair of hands to help you, an adult or close neighbor etc., or a senior citizen or relative, or close friend etc.
For me, I was simply amazed at how my daughter handled my being so pregnant and being not as mobile as I got closer to my due date. She was a real trooper.. .and I made sure we always talked about it and I emphasized being a "team" with Mommy so I don't have pains etc. My girl is also very bright and cerebral and active... but, sometimes instead of feeling like I "always" have to entertain her... I simply would explain that I can't and that I"m too tired, and that I need to rest otherwise "Mommy can't take care of baby in my tummy...." I had bad morning sickness too, and threw up, had migraines for 4 months.....so it was not easy. But my girl really amazed me. She somehow "knew" that I was not able to do things as I normally could. And I would explain to her that she needs to play by herself too, as I couldn't be running around.
Some activities we did: Puzzles, drawing, we also played "hide and seek" but with me sitting down & my daughter would hide and "verbally" I would "find" her and call out my guesses as to where she was, I Spy, or hiding objects in the room and then I would verbally call out where I think she hid it, play-doh, crafts, playing puppets behind my big tummy, and well educational videos when I simply was too tired to even think.
Sometimes, even having another person come over just for company and with the understanding that it is to give you a rest and give your son some added "stimulation" is also great. My friends/family understood that and it was less stress for all.
Good luck and take care,
~Susan