He must disclose his crimes, and they might require a waiver as well. There's no way of telling without reviewing his criminal records, though.
Waivers are very complicated, and you should definitely hire an attorney. Attorneys charge differently, and you should consult with different ones before picking one to work with. You can go to ailalawyer.com to look for attorneys in your area.
Do not file anything prior to retaining an attorney. The case could get a whole lot more expensive if an attorney is hired later on to fix mistakes that could have been avoided from the beginning. Good luck!
[This answer is for general purposes only; it does not constitute advice and does not establish an attorney-client relationship.]
Waivers are indeed very complicated and the need for one depends on a slew of factors so that you should consult privately with an attorney in a case as complicated as your fiancé's.
The answer provided is for general information purposes and cannot be relied upon. In order to provide legal advice, one must engage with a live attorney; this answer does not create such attorney-client relationship.
You should not be filing inadmissibility waiver without a competent immigration attorney on board. This is not a sale pitch, but an attempt to help you not have your waiver denied because you did not prepare or document it properly .
NYC EXPERIENCED IMMIGRATION ATTORNEYS www.myattorneyusa.com; email: firstname.lastname@example.org; Phone: (866) 456-8654; Fax: 212-964-0440; Cell: 212-202-0325. The information contained in this answer is provided for informational purposes only, and should not be construed as legal advice on any subject matter.