A longtime member of the Somerville Board of Aldermen won’t be running for reelection. Thomas Taylor, the alderman for Ward 3, said in a letter to constituents May 13 that he will give up his seat on the board when his term expires in the fall. Taylor, who has battled cancer in recent years, said health concerns prompted his decision. He has been a fixture in Somerville politics, serving on the Board of Aldermen for 25 years and previously spending three terms on the School Committee. “I love Somerville; it is my home town and I intend to stay concerned and involved,” he said in his letter.