If you're boyish, you resemble or act like a young boy. Your uncle's boyish features might keep him charming and attractive even as he grows older.
When you describe someone as having boyish charm, it means that person — often an adult man — has kept some youth or sweetness as he's matured. A boyish smile might be full of humor and mischief. The earliest meaning of boyish, in the 1540s, was simply "pertaining to boys," and it grew to mean "childishly silly," before gaining its current meaning.