Hi neilfin!!
I am so sorry that your son was diagnosed with HM, i to have it and was searching all over the world for more info about this sickness, beacuse it is very rare it was hard to find that.
But i started to write in this forum but then i found the best forum for me...www.healtcentral.com where people with HM from all over the world meets and chat on the messageboard.
I am from Sweden so i ca´nt give you any help with specialist from your area, but maybe some from the other forum can...so go and check it out too.
It is great that you already have a competent neurologist and if he is educated and have other patients with this disease then maybe he is a specialist then? Beacuse many neurologists
have never seen a case of HM.
Mine who is a migraine specialist had´nt had a patient with HM in 17 years before he saw me, and he is great and of course my country are so much smaller then yours,so that sure makes
a different too in how many cases of HM there is. But in this other forum many of us has experiences the same thing many have wated many years for their diagnosis and we are all so glad that we
found each other and can talk about our experiences there.
Hope to see you there 
Take care and i wish all the best for your family, i no how hard you have it now, beacuse it isn´t easy to live with and for family to be around theese attacks of HM.
Hugs from Sweden