Neon began his first round of radiotherapy on Thursday after completing a course of play therapy and a practice run on Wednesday to prepare him.
Sources said he was "doing very well", The Mirror reported
"I have seen pictures of Neon having the play therapy. It is heartbreaking," Mrs Roberts is reported saying.
During the court dispute, Mrs Roberts prompted a nationwide search after going into hiding with Neon.
He has since been living with his father.
Neon's mother previously suggested she would appeal the High Court's decision, but no appeal was made by her lawyers before Neon's treatment began, the newspaper reported.
Mr Roberts reportedly refused to allow his former wife to visit their son in hospital at Christmas.
Delays in Neon's treatment have caused increasing concerns among surgeons about Neon's chances of survival.
He had the cancerous brain tumour, called medulloblastoma, first removed in October and a second operation to remove more cancerous cells from his brain last month, which Mrs Roberts opposed.