Roger Speed said he and Gary’s two sons will be watching pitch-side as the team go head to head with Portugal in the Euro 2016 semi-final, but that it was too difficult for his mother Carol to be there.
The Leeds United legend was found dead in his home in Huntington, Cheshire, in November 2011 after rowing with his wife.
Speaking to BBC News in Lyon, Mr Speed said it would be a “very emotional game” and that he was in “tears talking about it”.
Asked what his son would say to the side, now managed by Chris Coleman, he added: “It’s fantastic. All he can say is, ‘Go out there and do your best. Do it for Wales. We are Wales, so do your best, that is all you can do’.”
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Mr Speed said he was very proud of what the team and his son had done, but continued: “Do not take it away from Chris. Chris has done a great job. I am behind him all the way for all he is doing. He has done brilliant.”
He described his son’s philosophy as “together stronger”, saying the team had bonded really well and that he felt some comfort when he saw the side was playing like a “team of brothers”.
Mr Speed described the Welsh side’s football as “coming from the heart”, adding that he would like to see them after the match.
Asked what he would say to them, he replied: “Well done, I hope.”
“I am very, very proud, really proud, and the boys are - his two boys. They are with me now.
“They will be singing with all of the fans, the pair of them. They’re good lads, lovely lads. They’re exactly like their dad, even when they are playing football as well.”
Speed’s career on the pitch included stints at Leeds United, Everton and Newcastle United. He was appointed manager of Wales in December 2010 after retiring as a player in May that year.
He took charge of Wales for the first time in February 2011, with his last game as manager on November 12 when the team won 4-1 in a friendly against Norway.
At an inquest in Warrington in January 2012, coroner Nicholas Rheinberg recorded a narrative verdict, giving the cause of death as hanging but adding: “The evidence does not sufficiently determine whether this was intentional or accidental.”