I'm sorry to hear your dog is not acting like himself and seems agitated.
From your description of him being attacked by a larger dog at the dog park, it's possible, as you suspect, that he either suffered some sort of neurological or other internal damage, although you see no visible wounds, and/or that this attack has caused him to be stressed and anxious and that's why he has become so clingy to you and your husband.
If he is refusing to eat or drink, there could be an underlying medical reason for his symptoms, or it could be psychological, but in any case, it would be a good idea to have him evaluated in person by his vet, for diagnosis and treatment, if deemed necessary.
So far, you're doing all the right things. Continue to syringe feed him water to keep him hydrated, and you can also give him nutrition this way. Water down some canned dog food and syringe feed him 1cc every 15 minutes for an hour, then repeat in 2 hours if he's still not eating on his own.
Continue to comfort him and allow him to stay close to you; also take an old t-shirt of yours and rub it on your arms, then pet him with it frequently throughout the day, so your scent is always with him. This may make him feel more secure. There is a dog appeasing pheromone, which may help him feel more calm, called DAP and it's available in major pet supply stores. It comes in a plug in diffuser, resembling a plug in air freshener, but has no scent to humans. Make sure to plug it into an outlet that is inaccessible to your dog.
You might also put some Rescue Remedy for Pets into his water. Follow directions on the bottle for dosage.http://www.entirelypets.com/rescueremedypet.html
I hope all will be well with your little furry boy, and you're able to have him evaluated by the vet, as soon as possible, to find if there's any medical cause for his current symptoms. Also, I'd stay away from that dog park, and/or keep him on a leash, so you can control who he interacts with. I know the purpose of the dog park is to allow him the freedom to have fun, but you don't want his confidence adversely affected, if he has another negative encounter, especially with the same Westie.