My sons top front milk tooth seems to have a really long root?

when he was 3 he banged it and it went grey. dentist told us it was because of damage to the nerve, but it's ok seen as though it's only the milk tooth. he is now 6, and it has been wobbly for about a year. i have recently noticed when wobbling the tooth, the other end of the tooth in the gum is protruding out of the gum. it has a very long root. is this normal?? it is just over 1cm long!