My son and the toilet

My son is 3 and 1/2. I can't get him to ever go potty when I want him to, ever, he is potty trained but just likes to go on his own time. He can hold it up to 10 hours plus at a time.

However, I'm pregnant and I have to pee all the time, I can't see how he holds it so long, I'm always telling him to go potty, he won't do it for me, I have to force him if I want him to go more than once in that 10 hour period.

Unless, I'm running towards the bathroom because I waited to long and my bladder hurts, and I'm screaming, "I gotta pee!" Then, my son seems to magically always have to pee, and cuts in front of me running into the bathroom just before I get there, and says, "No mommy it's my turn, I have to go potty."

Luckily our house has more than one toilet.