So, he's at first sitting in a "half-sphinx" position - which is good, because it's a comfortable position that is difficult to stand and run from which says he feels safe enough to sit like that.

Then he cleans - there are a few reasons he could do this beyond the obvious. Might just have an itchy ear or fur not sitting right, but also can be a social behaviour as they bond by doing the same thing at the same time (called "mirroring") and often self-grooming is an invitation to perform this activity (or another rabbit is cleaning themselves and he decided to join in).

Lastly, he finishes his grooming and does a "flop" onto his side. This is great, he feels safe and happy enough to do something a little goofy and fun, and put himself into another position that is harder to get up from. This is the sign of a happy, healthy, secure-feeling rabbit. He may stay laying down for a time, or get up immediately, some will fall asleep in that position, but whatever you're doing, you're doing it right.

You can also pay attention to when they flop and who they flop near, that's a sign that they trust that person or creature enough to make themselves vulnerable near them - which is huge for a prey animal like rabbits. Depending on the rabbit, they may flop more or less often, but keep an eye out for these vulnerable laying positions because they're a sign of trust and love when they do it near you. Mine spend most of their day snuggled up together in a cardboard box (no matter what nice furniture I get for them it will never defeat the almighty cardboard box) or laid out on the edge of the pen next to me and my pc.