Depends on a lot of factors. If your wife is a baptized Catholic, and no other marriages (that haven’t been annulled), then you can marry in the Church. But you need to talk to your parish priest or pastor ASAP and bring ALL of your documentation along with you. Tell him everything and follow his recommendations.
Whether or not his wife is Catholic is not relevant. He can convalidate his marriage to. A Catholic or a non Catholic in a large or small ceremony
A Catholic can marry a non-Catholic in the Church at any time (if they go through the necessary prep.) My aunt was away from the Church for some 15 years and had three ceremonies: a civil wedding (they eloped), then a Protestant ceremony, and then a convalidation ceremony many years later (so that’s when they “counted” as actually being married). My uncle was willing to have the marriage convalidated so she could return to the Sacraments, and other than a few meetings with her priest there were no major hurdles they had to go through.
Correct, he is in an invalid marriage that he wants to validate.
Because that is what a convadliation is. I am not understanding your confusion. He is in an invalid marriage that he wants to make valid-- so he needs to convalidate the marriage.