Hi, I'm sorry that your beardie has a lump in his mouth, it's going to be very difficult to tell you what it is without at least a photo of it, and I know it's hard to get a photo of the inside of their mouths. Even with a photo there is no way to really know what it is, as it could be a lot of different things...
How long has it been there? Did it just pop-up overnight, or did grow and get larger over time?
It could be an abscess caused by a bacterial infection, that's probably the most common cause of mouth growths, but it could also be a cyst or a tumor/mass. Whether or not it's malignant (cancerous) is anyone's guess.
I will tell you that you definitely need to find either a Certified Reptile Vet or a Reptile Specialist with lots of Bearded Dragon experience, and not just an "Exotics" Vet, because I can guarantee you that 99% of the vets you could take him to will open his mouth up, look at the lump, immediately tell you that it's probably an abscess, and they will just put him on an Antibiotic, probably Baytril, and that will be the entire appointment. They won't do any diagnostic testing at all to identify what the lump is, they will just assume it's a bacterial infection. Furthermore, even if it is a bacterial infection, they will automatically put him on a broad-spectrum Antibiotic, like Baytril, without doing any type of culture to determine not only what bacteria is causing the abscess/infection, but to also determine what Antibiotic the specific bacteria will be sensitive to. This is a MAJOR problem with "Exotics" vets, in general they don't have a clue what they're doing, most of them don't even ask you any questions about the Dragon's husbandry or diet. Then after you leave they go into the next exam room and see a parrot or a rabbit. It's a big problem.
Whatever vet you take him to needs to do a Fine-Needle Aspiration and take some fluid and/or cells from the lump, and then send them out to a lab for pathology and/or microbial testing. When they don't bother to do this and instead just put the Dragon on a broad-spectrum Antibiotic, a lot of problems occur...#1 Is that it may not be an abscess at all, it may be a cyst, or worse, it may be a tumor, and so not only will your Dragon be taking an Antibiotic that is totally unnecessary and will probably make him sick, but the actual problem won't be getting treatment, and then #2) If it is an abscess caused by a bacterial infection, without identifying what bacteria is causing the infection it's quite possible that the Antibiotic they give him is the wrong one and won't help, then not only will the abscess/infection keep spreading and get worse and worse, but your dragon will then have to take at least another round of Antibiotics, hopefully the proper Antibiotic, but even after a month with it not getting any better, many Vets still won't take a culture, they'll either put him on another round of the same Antibiotic, or they'll "try" another broad-spectrum Antibiotic, and maybe it won't be right either, and it keeps getting worse, etc. And the entire time all of these antibiotics are making your Dragon very sick, he's lost his appetite, he has no energy, etc. It's a vicious cycle that is very, very common.
If you can get a photo of the lump to post you certainly can do so for us to take a look at, but I recommend that you do a search for an experienced Reptile Specialist, actually we can help you find one if you give us your town/city, and when you take him for the appointment you insist on the vet doing a Needle Aspiration/Culture to be sent out to a lab for testing to get a definitive diagnosis, and do not allow the vet to put him on any medications at all until they get the results of the culture back. And stand your ground on that, a lot of vets will tell you it's unnecessary, let's just put him on the Baytril and "See what happens", etc. Just say no, it's your right as his owner.
In the meantime, you could buy some MaxiGuard Gel to put on/around the lump, it's an oral antiseptic/antimicrobial that may help it a bit until you can get him to a Reptile Specialist.