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im sorry hes being like this with you. maybe hes too ashamed to say how long it really had been going on for? sometimes "baby steps" is the best way to healing. if it were me, im not so sure how i would cope and deal with it. but taking a step back and seeing it how he might feel or think or his beliefs is the only way i can think of advice in this situation.
treatment is the only way he will get better. but when he does get caught in a lie, one could not engage in that further. that would lower the argueing and would also give him a sense that you dont agree with what he just said.
i dont think there is a quick and easy fix for this...
in my experience, the individual is usually sick, and its more of an addiction than a habit.
the fact that hes admited some of it to you, is actually a very big step.
with that said, keep doing what you need to so that you can stay safe, both mentally and physically.
he may not of realized this yet, but if it were me, i would make sure he was tested for STDs. and i would prolly get tested too to make sure that nothing was transfered during that time.