Could be a polish mix. Might be a mix of any number of breeds as well. Mini rex, mini satin, havana. I have some of those same toys, and judging size off it, it's definitely way too big to be a purebred polish (in my opinion, too big to even be one without the dwarf gene from lines that have the dwarf gene). It could probably theoretically pass as a havana, but those aren't popular outside outside of the show world. Without a history, all you can call it is a mixed breed.
It looks to be full grown in my opinion (the coat looks shorter and thicker like an adult coat), but beyond that I can't say.
At this age, if you can peek under it (with an open floored wired cage like that, you can try to stick your head under and look if you don't feel comfortable flipping the rabbit over yet) you'd likely be able to easily see the testicles if it's a buck. They're hairless and weird looking lol.
