Can you describe it in a few ways:
1) Is it covered evenly in fur or does it jut out, as if it suddenly appeared and the fur has been distorted?
2) Is it hard or soft?
3) Does it move or wiggle, or does it seem grounded to somewhere by your rabbit’s chin?
4) Google rabbit dewlap. Does it look like that?
5) While you may require your mom to go to the vet, you may be able to call the vet on your own time and leave a message with someone, telling your rabbit’s vet to call you back with any suggestions or ideas. I find many vets don’t default to “I’m not saying anything, just come in for an examination.” You can attempt contact, even if your rabbit’s chart is under your mom’s name. You’re her child and he’s your pet, so the office would not bar you from asking questions and expressing concerns on your own!
… Monitor his poop and drinking habits. If it’s not normal, this is a sign of something which may require vet attention.
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.