J.m, I know nothing about the availability of Ivermectin in the UK, though as you probably know, it's used as a de-worming treatment. In Australia it's available under the brand name of Combantrin (amongst others) and is an off the shelf, no prescription needed.
One of the big issues with alternative drugs like this is that there's been no testing to come up with a recommended dose rate, and because there's no proven value, any suggested dosage is random. If you can find a supply of this stuff you're in the awkward position of not having any scientific data to tell you whether the dosage should be one pill a week or half a dozen pills a day.
Please, if you or your son do find a source of supply, don't change the path of treatment that's been recommended for him.
Jules