"shall you lend him": If one has the means, but does not wish to fend for himself, he is given the money as a gift, and payment is exacted of him after his death. The sages say: If he has the means and does not wish to fend for himself, he is not attended to. If he is impoverished and does not want to take charity, he is told "Bring a pledge" (as security), in order to appease him.
"sufficient for his lack": You are commanded to sustain him, but not to enrich him.
"which is lacking to him": even a horse, even a servant (if that is what he is used to). And it happened with Hillel the Elder that he gave a pauper of high pedigree a horse with which to work and a servant to attend him.