Lump on neck of a three year old, grandparents making me paranoid, HELP?

My son has had this lump on his neck for over a year now and i have had it checked out by a doctor who said it was just a bit of gristle underneath the skin. Now his nanna and grandad are making me paranoid giving me leaflets about childhood cancers and stuff saying that thats what it could be because it's getting bigger (which i haven't noticed and he's with me more then them) and that i should get it checked out again etc. They are making me really paranoid about it, what do i do? Should i take him back despite already being told by a doc that it's a piece of gristle?