It's still rape if at any time during the act you withdraw your consent and he doesn't stop.
You need to communicate with him. All of your issues and concerns. Explain that you have some expectations he is not meeting, and that you are not ok with performing sexually in certain positions. If he keeps trying to convince you to do things you're uncomfortable with you need to stop that moment and leave the room. As long as he has your implicit consent and you stay quiet, he will not know how much it bothers you.
Drink water, and pay attention to your glass in general, with or without your boyfriend. I have heard of guys slipping viagra/cialis into their girlfriend's drink before, but it is so they are in the mood for sex more. It doesn't make it ok, however guys aren't always well known for being considerate.
Worse comes to worse, get a nanny-cam and set it up pointing toward your dinner table. Or get a purse-cam and set it on the table with your drink and look at the footage in private after. If your suspicions are confirmed, then you confront him and most-likely break up with him.
I would say that trust issues are probably the biggest concern in your relationship. Although it doesn't fit typical abuse patterns based on the information you provided in the OP. I have been through a lot of abuse, and I hope that your relationship doesn't spiral or escalate into abuse cycles. Feel free to PM me if you want further advice outside of this thread.
Take care, hugs,
- Katie