Katchie, my take is that if he was really working on his sobriety and making progress he wouldn't lie to you about AA. He might say it's not for him and he's more comfortable with (insert whatever else he'd be doing instead). Not everyone uses AA, I didn't, but I wouldn't have any trouble being truthful about that because once I began recovery the last time it worked for me.
If he's not recovering, he'd be likely to lie about AA. I don't see a scenario where he's actually in recovery but not going to AA because then he'd have no reason to lie. Trust your instincts.