Infinity is not a number; it's a concept. You say the teacher asked for the 'longest' number and told your son that the answer if nine 9s in a row followed by recurring. Surely ten 9s in a row followed by recurring is longer. Not too sure about this teacher.
Looking back at my response, I think I was a little too blunt towards the teacher. What I meant, I guess, was that the teacher should not give a definitive answer to it because there is none. It is not the type of question which has a single answer: such as "What is the present capital city of France?" The question he/she asked was meant to promote thinking, discussing, and researching amongst the students which it probably did: just look at our responses!
Dont worry, his teacher will never see this !!
I thought it was a strange question to ask, but i thought if i put it on here, i might get it explained to me.....
I think everyone has been very constructive and im really pleased will everyones comments.
If he gets another question that i can't answer, i will put it on here again, so keep your eyes peeled. Ha ha.
Thanks EVERYONE.