Have you tried benadryl as an abortive after the perfume? I have little success with abortives and although I do use Topamax as a preventative I can see how it would not be a medication that would be great for a young person. I’m not sure how old your son is, but Benadryl is very effective and easy to access with such small side effects. It is, after all, what they’ll put in your IV drip at the hospital when your migraine is so severe you end up in the ER. I think this may be a good option to try for perfume and chemical exposure migraine triggers.
Also, its good to remember that triggers are cumulative, so there may be other things building up in his system that you have yet to discover.
Perfumes are easy to control at home. If there are exposures at his school or other places he goes regularly, it’s perfectly acceptable to address the institutions with his health concerns and request a “no perfume” policy. It certainly can’t hurt to ask. I would say avoiding triggers is much more effective at migraine prevention than any pill. I wish you luck.