Property developer John McDonald, the businesswoman's son from a previous marriage, has died at the age of 51.
Mr McDonald, of Bridge of Weir, Renfrewshire, ran housing firm J&M McDonald Properties Ltd with his brother Michael, a director of Celtic Football Club.
The cause of death is unclear, but the family are said to be shattered by their unexpected loss.
The Weisfelds are best known as founders of high street chain What Everyone Wants, which was sold for £50 million in 1990.
They opened another store, Weisfeld's, in Glasgow in 1995 where Mr McDonald worked as managing director.
Michael McDonald has been a director at Celtic Football Club since 1994 and is one of the youngest members of the board.
He used to be one of the largest shareholders at the club behind chairman Dermot Desmond.
A spokesman for the club said: "The loss of John at such a young age is tragic news and the thoughts and prayers of everyone at Celtic are very much with Mrs Weisfeld, Michael McDonald and the wider family at this very difficult time.
"Clearly they will receive every support required from the Club. We would also request that the family's privacy is respected at this time."
Gerald Weisfeld opened the first What Everyone Wants in Glasgow in 1971. He and his wife then worked together to build up the business, which soon turned into one of the country's most illustrious and recognisable high street chains.
The couple firmly believed that the company should give something back to the communities that supported it and created the What Everyone Wants Charitable Trust during a period of expansion for the business.
Following the sale of the retail chain, the pair continued their charity work by setting up the Weisfeld Foundation, which John was also involved in.
The foundation co-ordinates work in Africa, Europe, America, and throughout the UK.
Mr Weisfeld is recognised as one of Britain's outstanding entrepreneurs and is a founder of the Entrepreneurial Exchange.
Mrs Weisfeld, who started out as a shop girl in C&A, was awarded an OBE in 1997 for services to retail.
Mr McDonald watched her received the honour at Buckingham Palace before her family celebrated with friends at a £100,000 part at the Gleneagles Hotel in Perthshire.
In 2010, the businesswoman was also listed in the top ten women who have had a major impact on Scottish society at the Action for Children Scotland Woman of Influence Awards.