I sympathise - I've only ever had two migraines, and from my experience they are a two-stage condition -
for the first hour you are afraid you might die ...
for the rest of the time, you are afraid you might not ...!
I don't believe they can be predicted, although regular sufferers learn to recgnise the early symptoms, before the main event kicks in.
If he suffers regularly (hope not of course) he needs to have contigency plans in place to get somewhere where he can lie down in the proverbial darkened room with some water, and a bowl in case he vomits - an occasional bi-product - and just sleep it off.
His GP may prescribe some strong pain-killers, I believe standard medication is not really effective.
Good luck - and here's hoping its just his hormonies, and will grow out of this dreadful condition.