Hmmm, the rescue should be responsible for confirming the sex and getting the rabbit spayed or neutered. That does look like a testicle, so perhaps the bun is young and they are just descending now? Some buns will also retract them when scared, so it can be hard to tell when you flip them. Do you know how old the bun is?
Don’t worry about treats, developing a good appetite for hay is very important.
As for bonding with the bun, the best approach is to play “hard to get”, meaning spend lots of time with bun but essentially ignore them. Let them explore without trying to pet or interact with them too much, this will help the bun learn you are not a predator trying to eat them.
Here are some litter box training tips, but the most important thing will be getting the bun spayed/neutered if not already (the rescue should be coordinating this though):
https://binkybunny.com/infocategory/litterbox-training/
.
.
.
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.