Hi Sunnybear,
meat with his mom and the other female JW and tell them in the first 5 minutes, that you have a deep relationship with her son including sex once a week (since half a year). You will see, what will happen.
Don't let yourself be forced into a dangerous cult whose supreme rules your friend doesn't follow. If his mom know that you have sex with his son, he has to be repent and he is being forced by the sect not to see you again.
If he does not abide by it, he will be excluded from the sect and his whole family and the entire congregation will no longer be allowed to have contact with him. This is the most inhuman thing you can do to a human being. It's cruel.
It's hard - but please run away from this cult, as fast as you can.