We're going to need more info to help determine why he isn't growing. While we wait for others to chime in please give us a complete run down on your care for this one. Housing including substrate, lights, temps, humidity, diet, do you soak? ect the more info the better
I have a feeling that this problem is nutritional. I have a couple of Manouria with the same beak. Be sure the tortoise is getting plenty of calcium-rich foods, sunshine (or UVB from an expensive light) and minerals to help the bones and shell grow properly and strong.
Once it happens, all you can do (besides correct the feeding/supplementation) is keep the beak filed to help it grow properly. Eventually, it will grow properly, but it takes a while to see improvement.
I suspect that your little guy can't chew his food and/or can't pick up much food. Try cutting his greens into small pieces and also try hand feeding. If he does well with the hand feeding, I would continue that until he's able to eat on his own and gain weight.
I have a subadult with a mangled beak who has difficulty eating and it's taken months and months of close observation, week weight checks, and experimenting with foods/textures/chopped vs minced, etc.
Here is what I have learned with mine (he name is Mo)...he was found wanting the highway and came to live with me.
Environment is everything. While nutrition is most likely the cause of his stunted growth, environment plays a MAJOR role and without the proper housing, you won't get anywhere with his health and growth rate.
The first 2 years (or until they are 5-6") they need to be in housing that replicates conditions for babies in Africa. Hot and humid dens. So a closed chamber, 80-90Â°, 80% humidity! and a PowerSun for UVA/UVB light with basking. If you are not using a PowerSun 100w, please earmark $40-50 and buy one on Amazon. Nothing is as successful in raising tortoises as a PowerSun. I tried others, they burned out or would not maintain the temp needed, so now I only by PowerSuns.
Without the above hot/humid conditions and correct UVA/UVB source, your Sulcata with not thrive.
Now my experience with feeding a deformed Sulcata. Here is Mo...it is presumed (based on observations from several VERY experienced reptile rescuers, that he most likely is the result of a bad beak trim.
Sadly he cannot eat hay, which is the bulk of an adult Sulcata diet.
He can munch on loose leaves from store bought spring mix, but they must be scattered on a terry cloth towel to provide enough friction so they don't slide around
He has a tough time with Mazuri, so I soak it, then form it into a few pyramids so he can bite straight down on them. He probably managed only half the volume I prepare. My large adult Sulcata thankfully finishes what Mo cannot, so he gets Mo's leftovers. They are houses separately.
The best success I've had is simply placing an entire head of random lettuces in his corner so he can push them into his mouth and they don't slide all over. I sprinkle Oxbow Critical Care Diet on the moist leaves for added vitamins/minerals and calories. (amazon).
Sorry about the typos. I really should proofread.
Also noteworthy is the fact that the malformed jaw is most likely genetic, and it's possible it's not the only genetic problem he has.