Outta curiosity, what makes you determine it's C8 impingement in the cervical spine? Why not the more likely ulnar nerve impingement further down (post brachial plexus) .. who knows, maybe some sort of elbow issue/bursitis that's impinging on the ulnar nerve there. If it were truly impingement at C8, he would likely have some sort of symptoms from the median, medial pectoral, medial brachial cutaneous, and medial antebrachial cutaneous given that fibers from C8 compose all of those nerves post brachial plexus. However, the only symptoms he's expressing is a cutaneous numbness in the dermatone supplied by the ulnar nerve, and not necessarily all of C8. And yes, obviously T1 extends some fibers to these nerves as well, but I don't see any reason to assume the entirety of C8 is compromised at the spine. Even this is all highly speculative.
there's a difference between nerve compression and nerve root impingement. as you're obviously aware, there's a ventral and dorsal nerve root that makes up the nerve. if the nerve root is being irritated by facet encroachment or an osteophyte on the ganglion or posterior aspect of the nerve, it could affect the sensory portion of the nerve and not the motor. it's mainly when you see osteophytes from the posterior bodies of the vertebrae on the nerve or nuclear material on the nerve or even a spondylo compressing the nerve that you have motor and/or sensory deficits.
furthermore, without doing an exam, it's hard to know whether he's got motor deficits in his finger flexion strength. the c8 involvement in a majority of the brachial plexus is really minor, except for the ulnar nerve. the other nerves kinda pick up c8's slack. furthermore, a dermatome is embryological and neurologically significant in determining where there's a problem along the spine. is it possible that his problem is elsewhere along the nerve's path (double crush syndrome)? absolutely. that's why i said to get checked out by a chiropractor, because i don't know without doing a history and exam. i'm simply speculating based on the info he gave me. it sure as hell isn't something life threatening, though.
i don't know a single person who doesn't know what a hangover headache feels like or know that it's pretty much something that'll happen if you drink too much. i'm making the assumption that the op is smart enough to know the difference between a hangover headache and a headache.
"mild migraines" are almost always tension (aka cervicogenic) headaches. it's really likely that if he has an issue in his neck that would be causing paresthesia at c8 dermatome that it could also be triggering a tension headache. that seems like the simplest explanation to me... not the off-chance that he banged his elbow hard enough to cause a neuropathy (without a bruise or tenderness of his right elbow) or and has a typical hangover headache (that he was dumb enough to not realize he would have after drinking).
maybe the op is dumb enough to have that scenario be what really happened, but i'm gonna give him a little more credit than that (even though his handle would suggest i shouldn't ).