Congenital Myotonia / Medication related fatigue/ Condition related sl

Hi Everyone
im writing from Australia regarding my 21 year old son who has congenital myotonia diagnosed at age 6 after many years of me being told nothing was wrong. In the end i researched genetic testing on the other side of the country and the paediatric neurologist said he would humour me and write the pathology for the tests which we were happy to pay the expensive cost involved for any answers. As it turned out i was right and the neurologist was apologetic and helpful from that day forward and helped manage his care as best he could with limited knowledge or experience of the condition and in our region.
he has been managed by an adult neurologist here in Newcastle NSW since he turned 18 who openly admits she has zero expertise and limited knowledge on the condition.
My son has transitioned from taking Tegretol for most of his life to now taking Mexiletine in the last 2 years and for some time enjoyed the results this change made. recently he has had to increase his dose and we have witnessed noticeably changes in his fatigue and sleeping patterns. the drowsiness is almost outweighing the benefit at this point.
i wondered if anyone has been down this road or has any new information regarding the condition as there dissent seem to be much of an update after all these year.