uh huh, after 32 years of relying on 'seeing how it goes' in sex I've been rewarded by a consistently shit sex life across 13 partners and a total of 0 orgasms. So I'm taking action and a different approach.
Good for you!
OK the best position for you (and him) will be revealed after experimenting with a few variations.
First thing to consider: do you want to be able to stimulate him at the same time as he's giving you oral? If so, you'll want to be in 69 (my least favourite position ever - both of you can't concentrate on either giving or receiving pleasure, but find out for yourself if it works for you!) In this case, get him to shuffle down the bed so you've got room to kneel with your legs on either side of his head, position your vulva over his face then lower yourself down and lean forward so you can either wank or suck his cock. He will need a pillow or two under his head so his neck is supported. NOTE: if you are very different heights this position may be unworkable.
If he just wants you to sit on his face and not stimulate him at the same time, that's easier. Again he'll need to shuffle down a bit. Swing your leg over his head and shuffle UP the bed til your knees are close to the headboard. If your vulva is now over his face, great. If not, he needs to shuffle down a bit more. Now you can lower onto his face and if you have a headboard you can grip it for support.
How much weight?
This depends on whether he's said he wants to feel dominated/trapped/smothered, or just wants to lick you without exerting much effort.
If the latter, then kneel, supporting your weight mainly on your own legs, with some support from the headboard.
If the former, gradually put more weight onto his face by opening your legs wider (while still kneeling) so that your pelvis gets closer to the bed. If you were holding the headboard, you might need to lean back.
If he wants full smothering (i.e. you make it difficult to breathe) then remember every 30 seconds to rise up a little for a few seconds to allow him to catch a breath before you resume full weight on him.
Remember to set a safe signal if you're doing full weight. A safe word won't cut it as he won't be able to speak (and him going "No, get off me, you're crazy!" might be part of the scene.) I ask my partners to tap my leg three times and I'll immediately "dismount". A visual signal isn't safe enough for me as I might have my eyes closed.