The National Institute for Health and Care Excellence (NICE) approved the medication for NHS funding in April.
Alexander has glioma, the most common type of brain cancer in children.
In addition to taking the medication, Alexander has undergone three rounds of surgery and radiotherapy.
When Alexander returned to hospital three months later after another seizure, scan results revealed he had a brain tumour.
'Immense challenge'Cameron Miller, director of strategy and external affairs at the Brain Tumour Charity, said the charity had previously funded research that helped identify BRAF mutations in brain tumours.

Bromley: Mother's relief as son gets cancer drug on NHS