The 71-year-old actor posted few pictures of the birthday bash on his blog. Agastya is the son of Bachchan and Jaya's daughter Shweta.
"Agastya's birthday... Enters his teen... Here to wish him... Back tomorrow," Bachchan posted.
The veteran actor, who was wearing a black jacket, seemed to be enjoying the November 23 party as he was seen giving a smile in most of the pictures.
The actor also posted a poem for his grandson along with the snaps.
"Luminosity on head and neck, joining the revelry on dancing deck, birthday dude is covered in hood, others bringing in the generous mood," the poem read.