Bobby Brown is celebrating the birth of his seventh child — and third with wifeAlicia Etheredge. Not long after the baby arrived Thursday, a video surfaced of the Roxbury-bred singer belting out The Temptations’ classic “My Girl’’ at Caffe Roma in Beverly Hills. Brown was apparently driving around town after the birth and, hearing a band playing, he decided to stop and join in. The 47-year-old Brown, who recently wrote a tawdry tell-all about his life, doesn’t look or sound much anymore like the R&B singer whose 1989 LP, “Don’t Be Cruel,’’ sold over 12 million copies. Brown’s other children include 1-year-old Bodhi, 7-year-old Cassius, 23-year-old Bobby Jr., 26-year-old La’Princia, and 27-year-old Landon. A year ago, Bobbie Kristina, Brown’s daughter with ex-wife Whitney Houston, died after being discovered unconscious in a bathtub.