The presenter opened up about the teen's diagnosis during a chat with Holly Willoughby and Philip Schofield on This Morning.
'He was finding school difficult and we hoped a new school would give him a whole new lease of life. He was struggling to concentrate and organise himself,' she began.
'His teachers were saying "He's a nice person, he's got a good attitude". But he was tired all the time. People were putting it down to the fact he's a 15 year old boy.'
As well as lacking in energy Fred - who was 15 at the time - suffered from crippling headaches.
After vomiting on one occasion and complaining that his vision was blurry, Philippa took her son for an eye test.
The optician found that Fred's optic nerves were swollen and said that he needed to have an MRI scan. The teenager had a seizure shortly after having the medical test the following day.
'The doctor said: "Your boy is really sick"', Philippa continued. 'I said, "Are we going to lose him?" And he said, "I think you are".'
The 49-year-old then told Holly and Philip that the family were flown to Salt Lake City by emergency jet, as they were living in the States at the time.
'The door closed and I just broke down. You've handed your baby over,' Philippa said of the moment Fred went into the operating theatre.
'We had a four hour wait, which was hell on earth. We wondered if we would ever get Fred back. Will he be the same person? Will it all come out? What are they going to find?'
Philippa was relieved to announce that Fred is now on the road to recovery, following successful surgery.
'It could recur, but it's unlikely. It was a super slow grower and now he will have regular scans. He probably had it for years,' she concluded.
'He's better and better all the time now.'
Philippa found fame in the 1980s while working as a kids' TV presenter for the BBC. She's since fronted shows including Robot Wars and The Heaven And Earth show.