Well, it certainly has a shell of some sort (thin surface layer, not just shape), and is somewhat egg-shaped, so that's a reasonable suggestion.
Please could you post some close-ups: I want to see if there is any pattern.
I can see it is in a piece of flint, and is probably made of flint itself, too. That makes me suspect it came from chalk, but the are many places where chalk crops out...
Where did it come from?
Hi Kaye and Mike,
I think it could be a shell because (presuming it's from the UK) no fossil eggs have been found (or I haven't heard of any.)
Hopefully someone knows more about it than I do.
In the images of your specimen I can see a small gap around were the fossil is attached to the rest of the flint matrix suggesting it was once covered in a thicker shell than what it has now.
Sorry it's not a Dinosaur egg but I think your son has found the internal cast of a sea urchin (Echinoid)