by definition, 1 gallon = 231 cu in. Calculate the size of the sun in cubic inches and devide by 231.
Incidentally, Google calculator knows many astronomical quantities, so you can just drop this directly into the Google search box:
4/3pi(radius of sun)^3 in gallons
to produce the answer.
Thanks everyone! I went with 372 Octillion, and when he asked what an octillion was, I told him we could write it when he got home. He said, “How MUCH is that” and I told him it was basically three million million million million million . I knew the was the place to ask!
Incidentally, in Little Dorky’s world, butter makes power spark and fizzle and sharks don’t like power because it makes their teeth feel funny 
Win.
Can i just say that site is fucking awesome? It’s gone straight in my bookmarks.
google calculator is better than that (though not quite as easy as wolfram alpha): diameter of the sun cubed in gallons returns 7.10997805 × 10[sup]29[/sup] US gallons
My 3 year-old son has a follow-up question:
Why?