How can I enrollmy handicapped nephew in a big brother program?

my nephew is a 24 yr. old mentally disabled with a mine of 8 yr. old. I'd like to have a big brother to spend time with him to teach him things and activities. He only sits in front of the TV and play video games all day. He needs interactions with others to develope his mind. he needs constant repeated instructions to follow direction. Also my daughter is a normal child and I 'd like to have a big sister for her too. Can you help in providing this sevices in the St. Petersburg/Tampa Fla. area?