Terry Brennan has been performing on Mondays at The Black Rose for nearly a decade. But this week, his stage was much grander. The 50-year-old Cape Cod resident sang “God Bless America’’ Monday at Fenway Park for the WEEI/NESN Jimmy Fund Radio-Telethon. He also revealed that he’d donated a total of more than $200,000 to the cancer-fighting cause over the past eight years. The singer-songwriter-guitarist said he was inspired to give after hearing a child speak during 2008’s telethon. “I was listening to the telefund, and [Red Sox announcer] Joe Castiglione was doing an interview,’’ he said. “It was about a 9-year-old kid who had raised money all spring and was donating it to the Jimmy Fund, and it just touched my heart. . . . I thought: With the gift that God’s given me, there’s got to be more that I can do.’’ Brennan, who grew up an avid Sox fan, said performing on the field was a “dream come true,’’ but he isn’t done supporting the Jimmy Fund. “I’m not going to give up until there’s a cure for cancer,’’ he said. “It’s a lifelong deal for me.’’