If you jerk your hand away or scream, you're just teaching him that his efforts to keep you out of his space are working. Rather than do this, you must ignore the bites (no matter how bad they are). If you can, get him to step up repeatedly, especially if he continues to bite your hands. Keep "laddering" him this way until he stops. When he stops, make a happy fuss and/or give him a treat. http://budgietalk.com/
(under budgie behavior)
Another way to prevent or decrease territorial behavior is to change the cage every week or so. Rearrange all the perches and toys, rotate in new toys. Not only will this help curb territorial aggression, it will also keep your bird from becoming bored. If your budgie seems possesive over a particular toy, remove this toy and replace it with something else. You can try adding it back in after a week or so has passed but if the aggression starts again you may have to dispose of the toy.
(same as above)
Dont know if that'll help at all =( If he's biting particular spots though, you could try putting a plaster or somethin on that spot till he stops. Just to prevent a bit of pain >_<