I ahve been in AA for more than two years and I do believe that in terms of sponsorship, it's best to keep relationships to the same sex. (My sponsor is a woman, I am a man, and it works but I can see why same-sex is suggested).
I see nothing wrong with supporting another member regardless of sex, but if there are issues in the past that make you wary, do share that with him. And if you are reading his texts and some mixed sex exchanges seem to be unrelated to the program, I would think that is a red flag.