Pregnancy can be just as hard on our children as it is on us. Mostly because they just don't understnd the diffence between long term changes and short term ones.
My first experience with this was with my 1 1/2 year old daughter during my final trimester with my son. She was too littel to grasp any of the changes so we recruited family and friends to take her to the park, the library, McDoanlds playland, a walk or anything so that I could lay down for a nap or ride out my daily Braxton Hicks contractions. It was a simple short term fix and when the baby was born all went pretty smoothly for them.
The challenge for us was when I was pregnant with our third child. My pregnancy was challenging, I was always exhausted even through the energetic 2nd trimester. around 24 weeks I was put on restricted activity because my Braxton Hick contractions were so strong there was concern labor could genuinely start any day. My son turned 4 during my 2nd trimester and my daughter turned 5, 3 days after I delivered. Once again, family and friend who could come and take the kids out were critical. But for behavior at home, the best thing we did was talk to them. I stocked up on books from the library about mom having a baby. our favorite became "Baby on the the Way" by Dr & Mrs Sears. We also intentionally watched nature shows about baby animals and tracked the arrival of new babies at the zoo.
It worked out for us that my kids like bears, and would always want to see the bears when we visited the zoo. Hard as it was to walk the zoo through Braxton hicks contractions, we did it only to find our favorite bear was not in the exhibit. Turns out she was pregnant too. So in the spring when we returned to the zoo with our new baby sister, there were two new bear cubs in the exhibit. 2.5 years later it is still our favorite must see exhibit at the zoo.
I have to disagree with the advice not to talk about the baby. The baby is coming, and staying. Your son will need to adjust to that reality. But how you talk about it matters. Include him in naming decisions, let him feel you belly when the baby is moving, bring him along to doctor visits to hear the heart beat. and if your doctor is good, he/she will enlist your son's help in taking care of you.
My 3-4 year old son became very good at rubbing my back through early contractions, so when the real thing hit and I was delirious , he was the one who held my hand while my husband called the midwife and we waited for the child care to arrive.
Good luck and remember, this too shall pass