Tips for riding- plus size

So I’m plus size. I’m 28, 5’3” and weigh about 380 or a little less. My boyfriend is younger and about 6’ not really athletic but not plus size. Im not confident in how I look. Can anyone give me some tips about riding him and being comfortable doing so? He puts in all the work and I really hate that he has to. I want to make him feel good too and not do the work. Last night I tried riding him for a little bit. I was soo nervous and took me about 5 minutes to even get the nerve to straddle him. Once I did I just kind of like rocked back and forth on him. He said it still felt good and that there was nothing to worry about and I’m trying to convince myself that it’s okay and just believe him. I want to ride him more. I struggle a bit with stamina, lower back pain and some knee pain. Any suggestions or tips would be useful. Thank you!

Update: I think your advice helped and was a success. He said he’s never got off to someone riding him (don’t know if that’s true or not but since he’s younger then maybe) Thank you everyone!