The 48-year-old actor - whose daughter Jane gave birth to son Jackson Riley in February this year - can`t imagine life without the youngster and admits he has given him a fresh outlook on life.
Jim gushed to TV show `Access Hollywood`: "I love being granddaddy, granddaddy Jim. It`s very odd, it`s not something I expected at this age to be.
"I just get a lot of joy, a lot of joy from that little boy - he`s just magical. It`s the greatest feeling in the world and as you get older - your connection to these things gets so intense - you just go, `Without this, I die.` "
The `I Love You, Philip Morris` actor - who split from Jenny McCarthy after five years of dating earlier this year - recently confessed he finds relationships "tough" and thinks he needs to have more respect for himself more in order to make his unions successful.
He said: "I`ve been obsessed and craved love, but always it comes down to the same thing. They are there to help you love yourself. When they go missing you go, `Oh, I see. I`m not done here. I still got a lot of work to do.`
"Relationships are tough. People are focused on saving themselves. Everybody is real motivated. Everybody is trying to realise a dream and I think everybody that comes into your life teaches you something and that`s it. Forever is an interesting concept. I don`t know what that means."