Humm, I'm thinking your question boils down to, "does the smaller rabbit have a dwarf gene?". I'm not sure if there is a genetic test, and I doubt you would consider it worth the cost. It is possible that "Shorty" does not. There other combinations of genes that could cause a rabbit to have small stature, that do not include the famous gene.
If Shorty does have a dwarf gene, how could it happen. Do you raise rabbits that are dwarf? If so it only takes one mating to happen, and as they say in Jurassic Park "nature finds a way". I suppose that one of Shorty's parents could have had a dwarf gene but the other genetics made the dwarf rabbit large enough that it did not appear dwarf. I suppose there is a possibility of a spontaneous mutation. I would consider this to be a one in a million chance, but...
Both parents could have carried two false dwarf genes, then at some point along the way one of those genes mutated to a dwarf gene. It is unlikely but possible. If you want to know would I would consider it a one in a million shot and not the less likely one in 10 billion chance then I can respond with a dry genetics lesion.
The TL
NR is, if you have a dwarf buck then perhaps he did get lucky. If you don't own a dwarf buck then it is unlikely that Shorty has a dwarf gene. If Shorty's mom and dad are re-breed and the occasional dwarf are produced, then it is possible that mom or dad has a dwarf gene, but the gene is "masked" by other genes. Meaning that mom or dad is genetically dwarf, but does not appear to be dwarf.