Obviously he has previously learnt that aggression is the best means of protecting himself and getting what he wants.
He might not have been badly abused to get like this. Just general punishing type treatment that most people wouldn't think was classed as cruelty can be enough to make a dog like this. Take Cesar Millans methods for example and how many times you see him get bitten.

You might find this page helpful. A behaviourist/trainer who does NOT use aversive techniques could help him. Any forceful methods used on this dog are likely to ellicit an aggressive response because this dog has learnt that when people show aggression towards him, he can stop it/prevent it by showing aggression back.
The right training can help him reaslise he doesn't need to show aggression but it's just finding it.

Here's the page btw:
http://www.dogwelfarecampaign.org/be...r-problems.php