eating and behavior

Question
Sorry, this may be long. Have 1 1/2 male wheaten terrier (had since 8 weeks) who from beginning was very picky eater because of me, i know. first time he didn't eat food, I put stuff in it to make appealing, which was worst thing I could do. He is currently on prescription diet from vet for skin allergies (venison and potato), but previously have changed his food many times, gradually, trying differnt ones to see if he would like one and also tried the leaving down for 15 mins and picking up rule, with no treats at all, and went about 3 days without hardly eating, maybe 1/2 cup in all. Then I gave in and added stuff again, and he gobbled it up and then after 2 days got tired of that and now not eating again. So need help with what to do about that problem. And other problem is he's aggressive toward other dogs at the park when he's got a toy or even if they come up to him to sniff him and he doesn't want to be bothered. He doesn't bite, but does that vicious warning growl toward them
and not really sure what the right thing to do is when this happens.  
If you have any advice on both of these problems, I would really appreciate it.
thank you very much  

Answer
With the food you must be aggressive in NEVER mixing anything into it and picking it up after 15 mins he will learn.Dogs will eventualy eat what they are given once they get hungry even though they might not eat for a couple of days,they will learn thats all they will eat

As for they way he is at the dog park,if he doesn't actually bite or attack the other dogs,it will be best to leave him,dogs are usualy smart if another dog growls they back off.But it won't hurt to use a water bottle,just in case another dog doesn't back down-and if it bothers you,take the water bottle and spray him with water and say 'leave it' when you spray him.Another method would be to just have him drag the leash and when he does that grab the leash,walk the other way and say 'leave it',but NEVER get in the middle of a fight,it would be better to use water if a fight happens