The comic was left devastated in January when the schoolboy was found dead at the family's Glasgow home.
Yesterday, Kiernan wrote on Twitter: "HAPPY BIRTHDAY SONNY! A TEENAGER TODAY!"
Friends and fans quickly posted messages of support to the star, who plays pensioner Jack Jarvis in the hit series.
Glaswegian actor Paul Donnelly wrote: "@FordKiernan1 thinking of you today pal."
Sonny, a Kelvinside Academy pupil, was found dead on January 6 at the family home on Cleveden Road in the West End of Glasgow.
He was found in the house where he lived with his father, mother Lesley, 44, and 14-year-old sister Kaye.
In April the 52-year-old suffered another tragedy when his mother Frances died.
The comedian is currently in preparations for his Still Game comeback. After splitting from his comedy partner Greg Hemphill in 2009, the two announced a reunion last year.
The pair are working on new material for a run of 21 shows at the SSE Hydro in Glasgow, which start in September.
The stage version of the popular BBC television comedy was originally planned to run for just four shows but demand for tickets resulted in 21 performances being booked at the 12,000-seat venue.
Ticket sales have topped £10 million.
On the first day of their sale, more than 100,000 were sold to fans, with demand leading to the temporary collapse of the venue's online box office.