It is nice that you want a Catholic wedding. In fact, as a Catholic, you are required to get married in the Church. Otherwise, this marriage would be invalid as well.
Has he signed up for RCIA yet? Since you indicate that he wants to become Catholic, one suggestion would be for him to start RCIA and to start the process of having his prior marriage investigated by the tribunal soon. Depending on the type of case it is deemed to be, he could have his answer by the time he is welcomed into the Church.
But the short answer it for both of you to go talk to your Priest ASAP. There are three areas where you need priestly advice (no internet advice) 1) the status of the “fiance’s” prior marriage, 2) his entrance into RCIA and 3) pre-cana preparation.