How can I bring my boyfriend back to the US?

When he left the US he triggered an automatic 10 year bar to reentry because of his overstay so he will not be eligible to return to the US until he has been outside the US for 10 years.

A fiance visa is not a good be option in my view. Your boyfriend has unlawful presence issues and for which, he needs a waiver if he is going to be able to return to the US. To be statutorily eligible for the waiver, he must show amongst other things, hardship to a qualifying US relative. Being just a fiance is not going to be enough for a waiver. But marriage to a US does. I suggest you file a Form I-130 after your marriage to him. He will undergo consular processing at a US embassy overseas. Prior to that, you should prepare a 601 Waiver of inadmissibility. You should not attempt this alone, and must instead hire an attorney especially with respect tot he waiver package.

It sounds like he is subject to a ten year bar for overstaying in the US for a year and then departing. I don't know if he also has any deportation/removal issues. But at the very least, you can apply for a fiancee or immigrant visa (when married.) Then you would need to file an extreme hardship waiver to overcome the ten year bar. Please have a consult with a competent immigration attorney to fully examine his case, to see if he has other issues and to evaluate your chances of success.

As long as he did not come back after leaving, then you can apply for a fiancé visa and a waiver of inadmissibility.

You should be able to bring him here on a fiancee visa or a spouse visa but more information on your situation is required to identify your best options. Consult with an Immigration Attorney and have him review his immigration paper work.