When I was preggers with my 2nd child, my daughter was 3... and turned 4 a tad after I had my son.
My daughter would still nap, at that age.
What I did (being pregnant), is I would spend a TON of time on her... 'explaining' in kid terms that "Mommy has to rest... to take care of my tummy..." etc. I included her into my pregnancy. It was not about me being pregnant, but her gaining a sibling. We would nap, TOGETHER at the same time. Even if that meant, me on the sofa, and her on the floor next to me. I incorporated her INTO my needs for rest/being pregnant.
AND I let her 'bond' with her baby brother in my tummy... she'd sing to my tummy, talk to my tummy, I'd take photos of her with my tummy each month or whenever she wanted. That way, 'my' pregnancy was special to her, too.
I am just saying in a roundabout way, what I did with my eldest, per naps, per my being pregnant, and she was that age.
And it helped.
I also explained, that even after her baby brother comes home... 'we all need to rest....' and have naps etc. So that, she 'knew' what was going on etc. So that, once her baby brother DID come home... she'd 'know'... what the routine was. With her, and with me and with baby brother.
I also explained that babies nap, too.
The thing is, yah, your son probably does not want to miss out on anything, being home with you guys on the weekends. But he is displaying tiredness and crankiness, per not napping.
So MAYBE if you 'explain' to him... like I did to my daughter... he will understand and not be so resistant.
It worked with my Daughter, when I was pregnant.
Then, once my 2nd baby did come home after being born... I STILL had a REGULAR nap time, everyday and 'our' routine. My daughter would then nap in the afternoons... when it was her baby brother's afternoon nap time. This was also after, my Daughter came home from Preschool.
I would nap too. I needed it. I explained all of that... to my Daughter. While I was pregnant and about how it will be after her baby brother comes home.
You can, explain things to a 3 year old.... in a way he will/can understand. So that he is prefaced about it all... and what is going on. Per your being pregnant as well, and for after your new baby comes home.
Explain it to him now... not after the fact.
K., need a head's up. Otherwise they do not 'transition' well, nor adjust to the new baby, after.
all the best,
Susan