If he can qualify for a work visa or a student visa, he could come over on that, on his own merit.
As KFDancer said, the Adult Dependant visa is extremely difficult to get because you have to prove that:
- through age, illness or disability, he is unable to take care of himself and relies on long-term, personal care to complete everyday tasks (washing, eating, dressing etc.)
AND
- even with your financial and practical assistance, he cannot get adequate care in the US because either it is not available and there is not person in the US who can reasonably provide it, or because it is not affordable