All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
As long as this person doubts committing an act of disbelief, he cannot be judged as a disbeliever, and thus his marriage contract is not invalidated. This is based on the Sharee'ah maxim stating that when a person embraces Islam with certainty, he cannot be rendered a disbeliever except with sheer disbelief. As such, his marriage is still valid and effective, and there is no need to search anymore for the issue of the “nullification of marriage by apostasy”.
You should furthermore know that none is obliged to adhere to a certain School of Law (Madh'hab) on all issues and that there is no thing wrong if the person imitates a School of Law other than his as long as he is sure that its opinion on a particular issue is more authentic than his.
Allah knows the best.