I had the same situation last summer with my oldest son (while pregnant with my now 7 month old). I was a working outside the home mom before then and when I became a SAHM, I thought "awesome, he and I will have a BLAST this summer)...then I got pregnant and was literally nauseous sun up to sun down. It got so bad, and my son had seen me vomitting so often, that he got VERY accustomed to patting my back while I kneeled near the toilet and would bring me water and crackers (I had them at the ready, so he would just go grab them).
Turns out, he was a very considerate and caring little boy and I really saw such a compassionate side of him.
Do you have any friends or family that can take pity on your plight and offer to pick him up every couple of days so that he has something more constructive to do than watch you be sick each day (LOL). I also got really good at coloring with him, all while keeping my head down on the table when it got too overwhelming.
Also, I recommend that you take advantage of the good times (be it an hour or two that you might feel 1/2 way decent) and make those moments count.
Most likely, even as young as he is, he is old enough to be empathetic and knows it feels "yucky" to be sick. Hang in there momma, better days are coming and you'll be able to feel better enough to join him in having a good time.
PS - Whatever you do, don't tell him "the baby is making mommy sick"...My SIL often told my son that and he ended up resentful of "the baby" for a while (until I finally got her to shut up and stop saying that). You can let him just know that mommy isn't feeling well and he'll likely be very protective of you (its very true that little boys are super protective of their moms, even at such a young age).