There is no "should" be out already. People choose to keep their sexual orientation private for a number of reasons. While it may make you upset that he feels that he needs to, for whatever reason, outing him is not the way. Have you considered that he might not be gay at all? Is this "don't ask, don't tell" environment one in which he would feel comfortable coming out? Or is it don't ask, don't tell because your family doesn't want to have to deal with the consequences of him being gay (mainly that they would have to face up to the fact that they might have some suppressed homophobia or, if not, that they would have to deal with other suppressed feelings).
Do you have an otherwise strong relationship with your brother-in-law? If not it would probably be appropriate to develop a friendship first, and to provide him with an environment where he feels he is supported and can be himself. If you see him struggling reach out to him, but let him be a complex person dealing with many things other than simplifying his actions and difficulties as all necessarily stemming from his sexuality.
If he is in fact gay and you two are close and you have made it known that being gay is a-ok in your book, he should eventually come out to you. But if he doesn't come out he probably has some very personal reasons not to. Unless he is gay-bashing as some kind of cover, whether he comes out is his own business.