ive noticed as my lab has grown, that only one big ball is there, not the normal two.
He is 6 months old, will the other ball drop down? or is he just going to have one?

Can he still be shown?

Yes it may still drop, yes he can be shown but will be penalised for the conformation fault and if it has not appeared by 12 to 14 months you should see a vet about it with a view to having the retained one removed.