They rushed him to Addenbrooke's Hospital in Cambridge where doctors told the shocked parents the bumps were a tell-tale sign of leukaemia.
Mr Tallowin, 32, said: 'It was more luck than anything else that we found it.
'Maddox had a Mohawk haircut. He has really big blue eyes and bright blond hair and it was a cut chosen by him a few months before. But it had got too long, about four or five inches, and it was beginning to flop so I decided to shave it off.
'The sides were short but when we touched the actual mohawk there were these bumps on either side of his neck at the bottom of his head. It didn't feel right so we took him to the doctor.'
Maddox's father found the bumps on October 14 last year and by the next day he was connected to drips and tubes and put on a course of chemotherapy to fight the childhood cancer.
His father, a taxi driver, said: 'Everything was so quick.'
Maddox spent a gruelling four months in Addenbrooke's, even celebrating his third birthday and Christmas in the hospital, before being allowed home.
Mr Tallowin, whose wife is a carer, said: 'The doctors said we had such bad luck at the start, he kept getting infections, like chicken pox, paraflu, and an infection in the line taking blood in and out of his chest.
'Maddox pretty much spent four months solid in hospital in one little room.'
The youngster is now in remission and the doctors have reduced the level of leukaemia in his blood but he still has to make long trips to hospital several times a week and will need chemo and intensive steroid treatment for three years.
The treatment has destroyed his immune system leaving the brave tot susceptible to infections and bugs so he is in hospital almost every day.
Mr Tallowin said: 'He is such a brave little boy and he has up days and down days but he takes everything in his stride. We are so proud of him.
'I feel so lucky we found it when we did. The doctors said that often it isn't picked up until the child is unwell and Maddox wasn't ill at the time so we got it early.'
Mr Tallowin said he and his partner have had to juggle their jobs around the vital visits to the hospital while also caring for Maddox's sister Elise, one.
He said: 'It's been an absolute nightmare - one of the worst things you could ever experience.
'But Maddox has always got a smile on his face. Luckily, both our families are very close by and they've helped us out a lot.'
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