Hmmm...I don't wish to come across as rude or preachy - but for Christ sake's man!
1. You are his mother's therapist! I would think as a professional, you would take into consideration that your flirting with and possibly having sex with her heterosexual son could adversely affect her treatment, and her health should take precedence over your hormones.
2. How are you "so sure" that actually intends on having sex with you? Maybe he just honestly wants to hang out? Why is that somehow unfeesable of an idea? Especially if you've lied and told him you are straight.
3. Dude...you went through his medical records!?!?! That's just plain wrong and, I would assume - or at least hope - illegal, any way you slice it. In any case, simply because it didn't say that he isn't carrying some STD, doesn't mean he isn't. He could have plenty of them and they just weren't detected at the time or even tested for.
4. (And this is just a personal thing of mine) - Why do so many gay guys keep pushing the issue with straight guys. maybe I'm a prude, but shouldn't "No" mean no? And if it doesn't, shouldn't "maybe" mean he's unsure and shouldn't be goaded and harangued into doing something he probably wouldn't want to do, or regret later? I absolutely hate when people I'm not attracted to persist to the point of irritation that borders on harrassment.
5. Why are you lying to him and telling him you are straight? Shouldn't he have all the facts before making a decision?
6. Justify all you want, barebacking is an idiotic practice in this day and age. You should know that. Everyone should know that by now.
Sorry man, if that sounds really harsh. But sometimes you need to quit thinking like a predatory animal and more like a civilized person....a therapist, perhaps.