How Do I Enjoy Sex With Severe Rheumatoid Arthritis?

Dear Betty,
I am a twenty-two year old virgin with severe rheumatoid arthritis. I've had a boyfriend now for a year, and though we have engaged in quite a lot of sexual activity, he is afraid that having sex with me will hurt me even more. I have extremely limited mobility in my left hip and right knee, and only so much stamina, but I love him and I want to try it. Even if it affects me poorly afterwards I'm still willing. Do you have any suggestions about what position might be the least likely to cause problems, and perhaps put my boyfriend at ease about hurting me?