I've never attempted deep throating, but I do have an easy gag reflex, so this advice might help...
When I've had to hold something large in my mouth, or have had to have the doctor stick a utensil back there for an exam, I've learned to close my eyes, relax my jaw and take deep breaths. I think if you try that and take him in slow and try to focus more on breathen and not on gagging...you might be ok.
Sorry I can't talk from actual deep throat experience, but I hope that helps some
ps: sweettuth, sorry to hear about your experience! that would suck majorly (no pun intended).