What stops people attacking my new coin?
Firstly, if he downloads the client wouldn't he have to sync with the existing block and be up to date.
From what I'm able to understand your concern is time warping (it should be but it kinda got solved in july 2010 for bitcoin if your coin is a fork of the bitcoin source). But to simply answer your question his solved blocks would have to get approved and even if they do they will get added to the network and synced to the current ledger. Therefore, only the blocks 10000 to 12000 would be solved by him and no it won't make it sync to his nodes.