Mr Willems suggested Sam write a letter, with the barbecue invitation, and he was surprised staff in Mr Key's office could even read "the schoolboy scrawl".
He told his son he would get some response, but was impressed to see a personalised card from Mr Key saying he had some understanding of what Sam was going through because he lost his father when he was only 6 years old.
Mr Key told the Herald his heart sank when he read in Sam's letter about his mother's death.
"That unfortunately is the reality of the great tragedy of Christchurch, that you have got youngsters growing up without parents.
"[Sam] was obviously a courageous young boy and I feel really confident that everything will work out well for him."
Mr Willems said Sam and his sister Olivia, 10, were resilient.
"But they certainly still do have their moments when the reality hits home, particularly around birthdays and Christmas times ... where they don't have a mum any more to be there with them."
The family were forced out of their damaged home after the February quake, but are living there again.
"Life will never be the same again," Mr Willems said.
"But the kids are a great support to me. And obviously they now look to me for all the support that they get. So between us we are doing not too bad really."