As the son of Dragon, Luffy will never be offered a Shichibukai position. The government wants him dead, not as an ally. Akainu views him as a global threat, similar to Ace, son of Roger.
Luffy has also committed the heinous act of embarrassing the government on multiple occasions. He embarrassed the government by defeating Crocodile, then he infiltrated Enes Lobby, survived a Buster Call, defeated Gecko Moriah, escaped Kizaru at Sabaody, infiltrated Impel down, lead a jailbreak, and survived the war after fighting against the Marine. That is not to mention the fact that he committed the unpardonable crime of striking a Tenryuubito. While Law also took part in the war he is an incredibly poor comparison as he is not the son of the world's most wanted criminal. To imagine Luffy being offered the post of Shichibukai would be to ignore the story, plain and simple. Nor would it be within Luffy's persona to accept such an offer. Accepting such a post would make him a subordinate of the government when Luffy quite clearly takes orders from no man.
Luffy being offered the position of Shichibukai would contradict all prior events of the manga. Accepting would require Luffy's very personality to be invalidated. For those seeking an argument to the contrary, the alliance with Law would be a horrible counter example. Luffy has quite clearly established himself as the leader of the alliance, already forcing Law to do his bidding. Law had no interest in saving the children, preferring instead to initiate his plan. Luffy made it quite clear this was unacceptable, that freeing the children came before Law's plan. Law had no choice but to consent, following Luffy's orders.