Will Ferrell's two-year-old son loves telling people they have a ''little penis''.

The 'Campaign' actor says Axel - his third child with wife Viveca Paulin - is being encouraged by his older brothers, Magnus, eight, and five-year-old Mattias to insult random strangers, and while he is trying to discourage his behavior, he admits he finds his gags very funny.

He told 'Late Show' host David Letterman: ''The two year old unfortunately is just wandering into vulgarity - he has two big brothers, it wasn't from me.

''I'm trying to prevent it. I'm doing everything humanly possible. I'm having a good laugh and it's out of my hands, I can't control it.

''He thinks it's hilarious - and so do his brothers - to walk up to anyone, male or female, and say, 'You have a little penis.' He'll get on a roll and say it over and over again.''

Will is even planning to use the youngster's love of the insult to play a joke on some showbiz pals.

He explained: ''My brother-in-law wants me to have a big Hollywood party and invite loads of actors, then hopefully he'll walk up to a George Clooney type and say 'You have a little penis'. He thinks it would be really funny.''

Meanwhile, Will revealed his 'The Campaign' co-star Zach Galifianakis gave him a generous gift for his recent 45th birthday.

The 'Hangover' actor presented his pal with a check for $5 million - but then was told to ''wait to cash it''.

Will added: ''There might be a hold on it.''