declaration of nullity

my live in partner for 8yrs is married before, both parties are 18 yrs old and no marriage liscense issued that time when they got married. they only have an affidavit that they were living together for 1 yr n 4 mos which is not true, he signed in that affidavit for them to got married because the girl was pregnant he had no idea for any papers that he sign. he did it just because of his mother without knowing what it is all about.can he use lack of marriage liscense for nullity of their marriage? how can we know if his marriage is valid or invalid? how to know if they have marriage liscense or they dont have. what if the other party (ist wife) has her own family now with 2 kids, what grounds can he use for it.. thank u!