Answer by taking a quote from the following article:

Hubert Neal McGaughey Jr. was born on July 30, 1958, in Jacksonville, Texas, to a Filipina American mother and Irish-American father. Inspired by the variety of music that his parents listened to, which included country, rock, disco and R&B, McGaughey first sang in his church choir before founding an R&B band. He later switched his focus to country music, performing in various bars and clubs in Texas. McGaughey, after attending junior college near his hometown, found work selling shoes at a shopping mall.

He then signed to Atlantic Records in 1990, changing his surname to McCoy per the label's request, as fans had already begun to refer to him as McCoy. His debut album, At This Moment, was released that year. None of the three singles made the country Top 40, although the lead-off single "If I Built You a Fire" was a Top 20 country hit in Canada. The other two releases were the title track, a cover of Billy Vera and the Beaters' hit song from 1987, and "This Time I've Hurt Her More Than She Loves Me", which was co-written by Earl Thomas Conley and originally recorded by Conway Twitty. McCoy continued touring and developed a "reputation for exciting, freewheeling live shows."  A second album for Atlantic, Where Forever Begins, followed in 1992. This album produced his first American Top 40 country hit in its No. 40-peaking title track, followed by "There Ain't Nothin' I Don't Like About You" at No. 57, and "Now I Pray for Rain" at No. 26. The album was also his first entry on Top Country Albums, at No. 58.  Working with producer Barry Beckett for the first time, McCoy released No Doubt About It in 1994. The album produced his only number 1 country hits in its title track and "Wink", both of which also made minor entries on the Billboard Hot 100. The album also earned a platinum certification from the Recording Industry Association of America (RIAA) and gold certification from the Canadian Recording Industry Association (CRIA). Its final single was "The City Put the Country Back in Me" at No. 5.

Why did he have to change his surname?
McCoy per the label's request, as fans had already begun to refer to him as McCoy.