He doesn't appear to have rex fur, so would not be a mini rex even if he is the right size.
Netherland Dwarfs do have the dwarfing gene, so about 25% of them can not have the gene and end up a bit bigger than the standard. He could still be a purebred, just doesn't meet the breed standard. He could be a mix as well, it can be hard to tell.
Does he have a collar on? Collars are not safe for rabbit and not needed. Anything just around the neck is not a good idea, especially if left on unattended. If you did need something for when your rabbit is out, an H style cat harness is best, but only when they are supervised.