
Hi,
Try not to panic. This isn't the best thing that can happen to a dog, especially not a sensitive breed with an elephantine memory like a dalmatian, and not at this particular stage in his life (adolescence). When did this attack happen?
I would try to make sure he only sees dogs that he used to be friends and play happily with, and avoid anywhere that he's likely to meet others just yet. With any luck he'll remember his friends and start to relax in their company. When he reaches this stage (can take weeks, so don't rush it) you can start to extend your range a bit.
Whatever you do, don't scold him for trying to 'see off' other dogs. This will only confirm to him that bad things happen when they are around. When you see another, try to pre-empt his reaction and tell him to 'sit'. Get him concentrating on you, then you can praise and reward him. If it goes to plan he will start to associate seeing other dogs with getting a treat from you - positive association! (Also a sitting dog finds it very hard to bark fiercely ;) )
If he hasn't improved after two or three months you should consider contacting a reputable behaviourist. But be prepared that your dal may never trust any GSD again.
Hope this helps.
:)