There are a number of things that could happen next, and this will depend on the exact diagnosis of the mass. It is likely your vet will have the information on what the mass is, whether it is benign or malignant, and whether the mass has spread anywhere else in Mojo's body.
It is likely that you will have to put Mojo to sleep eventually - but hopefully this is quite some way away. It will ultimately depend on Mojo's quality of life, how much pain and irritation the mass is causing to him each day, and whether any further complications develop as a result of the mass (infection, or spread to other areas of his body). Because the mass can't be removed, it is likely it will enlarge, or that the area of ulceration will worsen over the coming months. So long as Mojo is eating and drinking ok, getting around the house as normal and interesting in play/exercise, and doesn't seem to be in pain or distress, then you are absolutely fine to continue, along with the plans for dealing with his mass in the meantime. When the time comes that Mojo's quality of life is really being affected however, then it will be fairest to have him put to sleep. Unless this mass is highly malignant (your vet will be able to let you know that information), then it is unlikely Mojo will die 'naturally' because of the mass. Therefore, because the mass will likely have an impact on his quality of life eventually, then eventually having him put to sleep will be the most fair option here.
Hopefully that is months, or even a year or two away! It will be a matter of taking each day as it comes and doing your best to keep Mojo happy in the meantime.
I hope this helps!