My guess is that you are losing your teeth from extensive oral sex. Maybe you are not doing it right. You cannot have any effects from his drug use by that manner.
You have some decisions to make as to whether or not you want to stay with this man. Best of luck.
There is very low risk of contracting AIDS from fellatio. There is also a theoretical possibility of catching HCV from oral sex. Simply using heroin on it's own is no guarantee that he has either. If he does, unless you have cuts in or around your mouth, you're still highly unlikely to catch either. Actual penetration ups the odds. If you're concerned & you're doing this on a regular basis, I'm hoping you're comfortable enough with this person to simply ask. If it's a 'casual' thing, I'd just drop it if I were worried. Frankly, your risk of contracting certain other STD's from oral sex are much higher.
Here's a link:
http://std.about.com/od/riskfactorsforstds/a/oralsexsafesex.htm
Also, get thee to a dentist if you're losing your smile :))