Hello, poor little guy, that's probably sore...I can't tell if it's an injury or an abscess/infection that just popped up, but how long has it been there would you say?
You say you have to "assist feed him", so I'm guessing that means that he refuses to eat on his own? Does he eat anything on his own? And how long has it been since he stopped eating on his own?
The reason I'm asking is because it's possible that the reason he stopped eating on his own to begin with is because his mouth hurt when ate, it's not the first time that a dragon stopped eating on it's own and the owner went over the usual causes of them losing their appetites, such as inadequate UVB or incorrectly mounted
UVB light (#1 cause by far), or improper temperature zones, etc., so they they went to the Reptile Vet and had a fecal test done for parasites, that was negative, but the Reptile Vet found an injury or an infection inside their mouths that was the culprit for the lack of appetite. Either way, it's not at all normal for them to need to be force-fed or assist-fed for a long period of time, if they do stop eating on their own for a long period of time there is definitely something wrong that's causing this, and the area in his mouth could very well be the reason he won't eat on his own anymore...after all, how often do we look in the back of their mouths?
Either way, I'd be finding a Reptile Vet that is experienced in Bearded Dragons and getting some cultures taken to check for a bacterial or fungal infection ASAP, as you don't want it to turn into an infection that starts to spread throughout his mouth, tongue, into his throat, or starts effecting his teeth. When a mouth infection starts spreading throughout the entire mouth/throat/tongue it typically will kill all the tissue along it's way, and this is what is called "Mouth Rot", and it's very hard to treat once it gets to that point. So it's definitely best to get him to an experienced Reptile Vet ASAP, while the area is still small and contained to the back corner of his mouth.
The best bit of advice I can give you for when you take him to the vet is absolutely do not just let the vet simply look in his mouth and then prescribe a broad-spectrum antibiotic (usually Baytril)!!!! This is pretty much ALWAYS the kiss of death and the start of a very frustrating, expensive, and sometimes fatal downward spiral. They refuse to do a simple, inexpensive culture to diagnose the exact microbe that's causing the infection (whether out or inexperience or just laziness) and they simply say "Here, give him this Baytril or other broad-spectrum antibiotic for 2 weeks", and so the owner gives it to the dragon, it doesn't work, they take the dragon back to the same vet, and again they fail to take a simple culture and either give them a second round of the same antibiotic, or they try a different broad-spectrum antibiotic for another 2 weeks. Again this does nothing to help, and by this time the infection has started to spread throughout their mouth. And usually by the third visit the vet will finally take a culture, send it off, and then they find out that it's either some bacteria that is only sensitive to one or two very specific antibiotics, or it's not even a bacterial infection but rather a fungal infection that was actually made much worse by the 2 rounds of the broad-spectrum antibiotics...So please, insist on vet trip #1 that the vet take cultures to determine exactly what microbe is causing the infection and what medication will treat this specific microbe (whether it be an antibiotic or an antifungal, etc.).