This is my opinion, and based on my experience. Please take it for what it is.Â
If your biggest concern is "it's going to hurt", you might want to consider the "cowboy" position, as it puts you more in control. You can straddle your partner, facing him, squatting over him, taking in his penis as slowly or quickly or deeply as you choose, and you get to see the look on his face. If you need to lower the intensity for yourself, you can lean forward for a kiss, hug or to suck/nibble/lick on whatever bodypart is exposed that appeals to you most and is in reach, be it his ears, nipples or hairy armpits. ;D
Also, that butt of yours is home to the largest muscle in the body, the gluteus maximus. So, even if you aren't buff enough to bounce your pecs, you should be able to squeeze his cock as you rise up, then relax your butt as you slide down, grinding him in such a way that should make up for any negative feelings he might have about not being a "controlling top" during the fun.
It's sex. Ideally, it's supposed to please both of you. You can also jack off a bit while you're doing it…or, if you find that it's more pleasurable than you expected you can keep your hands off yourself and all over HIM. :hug2:
Obviously, there are many other options to try once you're more "loose" or "experienced". But, if pain is your worry, I think this position gives you the best chance of making his first visit into your ass a welcome experience for you. And, afterwards, you can probably try other options with far more pleasure than pain. And, if talk to him and ask him to be gentle, I'm sure he'll work up slowly, rather than pounding you into next week and having your co-workers as why you're walking funny the next day. (Of course, you could do this the night before your day(s) off, so they never have to know.)
PERSONALLY, I much prefer any position where I can see the man I'm with, no matter which of us is "top". I want to see his reactions…and have him see mine.
As a closing thought, there is the remote chance that you won't enjoy it as much as he does. If that's the case, you'll need to decide for yourself whether that means "the door is closed", or you're willing to let him top you occasionally to make him happy.