This is SSDI, not SSI. He's still working, so should continue to work until the SS Disability is approved. Chances are the immigrants will already be in the USA by then, making SSDI income for sponsorship a moot point.
Yes, if SSDI is enough, it can be used by a petitioner to sponsor an immigrant if it's enough. Since it is not a part of the present tense of this case, that really doesn't matter.