Sorry you are going through this at the moment, my DD is 18mths old now and I have been through this on and off since she was 10 months. I dont really have any advice except do you think maybe he senses you are pregnant. Maybe try taking a drink of water into him with you, if I do that with my DD she usually drifts back off to sleep easier, otherwise she does what your DS does and I think she is asleep go to sneak back out and she say "mum" and I have to start all over again.
Good luck I hope it fixes itself soon for you and congrats on your pregnancy.