The celebrity chef can be seen staring down at his son looking slightly confused about why River is so excited.
In front of them is a large book - that appears to be a cookbook and is apparently really funny.
“Cosy New Year’s Day and the boys decide to do a little light reading,” Jools captioned the shot on Sunday 1 January.
As well as commenting on how gorgeous River’s smile was, fans were doting on his Rocket pyjamas - in reference to his middle name.
“That is so precious and I love that River Rocket wears rocket pyjamas, a no brainer,” one person wrote.
Another commented: “Oh my gosh you can tell he is proper giggling, I love it when they do that at the simplest of things.”
River is the couple’s fifth child. They are also parents to Poppy, 14, Daisy, 13, Petal, seven, and Buddy, six.
Jools has been sharing snaps of the whole family spending quality time together over Christmas (and that of course includes plenty of cuddles with River).