I'm wondering too if it's possible he's not full-blooded. If he could possibly be a mix, then it's especially true that you can't necessarily go by weight alone. I have a Jersey Wooly who is 5 lbs. JW's are supposed to be 3.5 lbs max, and to my knowledge she is purebred (has a pedigree), but apparently she didn't get the dwarf gene, so she's oversized. She isn't fat though because she has good definition, and the vet checked her and said she's just right. So anyway, I'm just wondering if it could be something similar with Shorty? Is he longer than his sister, or about the same length?
If he is overweight, I'm sure the vet will have some suggestions. Try not to worry. I'm sure the vet won't think you're a bad bunny mom since you've had him on a diet and are bringing him in to be checked. Good luck tomorrow! I hope everything goes well. Let us know what the vet says.