I would just say, "no really, thanks very much but I can't drink....it interferes with:
A. my meds,
B. my doctor told me not to drink with my meds
C. I've been so sick, the thought of drinking this beer would make me puke all over your nice rug/floor/wife/dog.
He's the A-hole for not respecting your wishes, not you....in my thoughts, it's just easier to play the above scenarios, but he is making a difficult situation worse for you.
would this person offer you something to eat, then if you refused, shove a burger in your mouth?