Hi and welcome!
I can understand wanting to get an answer rapidly, and so posting on an internet forum. But, you need to go talk to your priest about the situation. Internet answers can be unreliable. Both replies to your question thus far are inaccurate, but certainly well intentioned.
The Catholic Church recognizes the marriages of non-Catholic Christians as well as the unbaptized as
valid. If both parties are baptized, the marriage is valid and also a sacrament. If one or neither are baptized, it is a valid natural marriage. So, yes, the person you are interested in marrying is already married in the eyes of the Church, and would require a full investigation of their marital situation.
It is not accurate (post 2) that it is “less complicated” for a non-Catholic to receive a decree of nullity, nor is it accurate (post 3) to state that the Church would view it as “purley civil.” Civil versus religous ceremonies for non-Catholics are irrelevant. In all cases the minister is acting purely as a civil witnesss. These marriages are
valid.
There are MANY factors that can influence the validity of your non-Catholic boyfriend’s marriage and the way in which his marriage can be examined-- for nullity or dissolution of the bond. These factors include any previous marriages of his ex-spouse (and any previous marriages of any of her previous spouses), the baptismal statuses of the couple, whether either party was every Catholic, and many others.
So, your boyfriend needs to go sit down with a priest and lay out ALL the facts. The priest will guide him from there.
Also, if he is interested in converting (not a requirement for marriage to a Catholic, but certainly a wonderful thing) he should contact the RCIA cooridinator and also make that known to the priest when he is discussing his prior marriage.
I can suggest you get a copy of the book Annulment: The Wedding That Was by Michael Smith Foster. Read it together, it will help you understand the Church’s teaching on marriage, nullity, and the dissolution of the bond procecesses called the Pauline and Petrine Privileges.
And, lastly, this chart may be helpful, especially after reading Smith’s book:
diocs.org/CPC/Tribunal/Decision_matrix.pdf