You poor thing- you must be exhausted!! First of all, congratulations on your pregnancy. And also, congratulations on your son being such a good night sleeper! 12 hours straight through the night is WONDERFUL, and so good for him. Nice work!
He does need to nap, though. Maybe only once a day, or maybe twice. (It sounds like he'd be ok with just an afternoon nap.) I know it sounds harsh, but I think he may need to cry it out in his crib. Maybe set a 45 minute time slot, and just put him down. Keep his room very dark and quiet. And he will probably throw a complete fit for a few days. But you need to rest! And when the new baby is here, you will have set the stage for your son to have at least a "quiet time."
He may surprise you, and fall asleep eventually after all. It may take a while, but it could happen! And when he's a little older, he can choose to nap or read quietly in bed. But you'll have the down time, which you'll need.
Good luck with the naps, and with the new baby in January!