It is not at all a great idea to lay your bun on his back, especially since you say he needs to be distracted with petting, still. They may look calm like that but it is not really appreciated by them. Usually they will kick out, twist, bite, whatever-shows their obvious displeasure.
Storm (darker lop on pfp) is a hard fellow to cut nails with, when we first went to the vets his nails were long and sharp and he was super fussy. Took the vet, his helper and me to somewhat hold Storm steady enough for a quick trim and we all ended up scratched.
He's calmed his temper a little bit (neutered and a fulll adult) but we still have a hassle with it. Usually my dad is able to help with it, but sometimes not.
The way we do it is as follows:
see when our boy is in a good mood, preferrably during high noon, when he would just be falling asleep.( I like to believe that he is a bit too sleepy to protest as much. I won't even try when he's zippy and hooky from his energy spurts.)
Entice him with some pellets or vegs for additional calm (he's not that fond of me)
gently roll him to his side, applying mild pressure near his shoulders w super spread out fingers to more evenly distribute the weight, the same goes for the hip area. Always mindful to be gentle so that he would feel least amount of discomfort
The other pair of hands will try and clip his nails, being careful to keep all and any appendages away from his face. He bites strong and is not happy in the slightest.
We do one foot at a time, letting him calm down and recieve a small treat in between. if we're not so lucky, it will happen between every two nails or so.
If Storm decides that enough is enough, and will not calm down enough, we ain't pushing it. Better luck next time.
He does have half-translucent nails though, so it is not so hard. I could not imagine a big bunch of sass like him having black nails. THAT would be a nightmare.
I hope that my experience may be of some use to you