It’s the sugar content in carrots that makes them a food that should only be given in very small quantities. Consider carrots as you would fruit, since they are all high in sugar and should be limited to occasional small pieces. The sugar can cause gas and GI issues. If your bunny was grinding his teeth and not eating pellets, it sound like he may have gas or be going into GI stasis. It is good that you are taking him to the vet today. It’s also good to keep infant simethicone drops on hand, in case of gas. They are perfectly safe for rabbits.
Here is the section at Binky Bunny dealing with a healthy diet. https://binkybunny.com/BUNNYINFO/tabid/53/CategoryID/2/PID/940/Default.aspx. There are links there showing good and not so good veggies to give to your rabbit, and it talks about fruit too.
I assume that the vet will do a fecal test. I had a past rabbit that we found on the street. She was having diarrhea and kept cleaning herself. It turns out that she had worms, which was easily treated. She also had the less severe kind of coccidia, which was easily treated. It seems like your bunny may be having issues due to his diet, but hopefully the vet will figure it all out and get him some treatment if needed. You really need to stress to your mother that a rabbit has a delicate, complicated GI system, and the wrong foods or too much of them, can cause big problems.