Savannah (Ga.) Calvary Baptist Day three-star defensive lineman Walter Mathis Jr.took official visits to Clemson, Miami, Auburn and LSU in May and June. That last visit to Baton Rouge made a lasting impression. The 6-foot-1.5, 285-pound recruit was on his second visit to LSU and that was all he needed to see. He has known his decision for some time, but he kept it to himself until his mother’s birthday on Aug. 31. He’s now ready to let everyone else in on his decision. “I wanted to surprise my mother,” Mathis told On3 about his commitment to LSU. “She didn’t know anything about it. It was a to…