There are so many possiblities of what may be wrong and fit the description of what you describe.
First off does the dog lay in the yard? Is he laying on ants? I've had dogs have reactions to ant bites tht sound like this.
Second have you seen any fleas? I had one who was alergic to the flea bites and if she ingested them, huge patches of hair loss, also if they do ingest a flea or mosquito you may want to worm the dog...yet another possible cause for something like this. Do NOT use a flea collar, they are not very good for anything but cutting up and putting in the vacumb. It is a massive amount of chemicals in the neck area that are absorbed into the skin and don't do anything for the other parts of the dog. If you are using a heart worm monthly pill, you may also be covering for fleas and in that case it would also over do the chemicals in the dogs system. Yet another possible explanation.
Third, did you change dog food gradually or just boom one brand then something new for the next meal? Check all your dog food and get your dog off of Soy to start with, yes there are dog foods out there with out, I've been using several for the last 2 years. They are more expensive but they are much better for the dog and most tend to feed less of the more expensive foods. Now here is the other part of it, some dogs are allergic or have reactions to a specific type of meat or just one ingredient in the various bradns. I know several people who have their dogs on duck and sweet potato or venision and sweet potato products because of yearly allergies both related to their diet and those not related to the diet. When you do change the diet make sure it is a gradual change, mix the two foods together and slowly add a little more of the new food to the mix at a time. The other way to do it is fast the dog for 24 hours, it helps the stomach with the schock of change. I sounds bad, but you need to get the other food completly out of the system then change. Also it wouldn't hurt to give the dog some yogurt to help replace the good bactera and funa that is supposed to be in the stomach which would have been killed off by the anti-biotics. When ever my dogs are on them, I make sure they either get a food enzyme tablet or some yogurt to help with digestion and to help the immune system.
Next what are you actually using to wash the dog? Anything by Heartz or Seargents really is not good, they are not tested to be safe the way things are tested for people. Water down what every you are using a bit helps to move it through the coat and to actually rinse out when you are done. Use a mild shampoo, I prefer Pert, it works wonders on coats and also helps to keep down fleas incase this is the problem. (not sold as such, but the dog show community has known this trick for years). I also use Garnier on my daughter's Chinese Crested's coat along with Aussie on my other dogs. I also try during the summer to use and mix the shampoo with something which has an orange oil in it to help keep away the possiblity of Fleas, with the travel we do. Bringing home a flea would be the kiss of death to me...they are horrible to get rid of and deal with.
And finally the most important...YOUR VET. I can recommend mine, he is more interested in the health of the animals then the money. The Pet Doctor on Hayse in Clinton TWp, is wonderful. I have used Dr. Syed for over 14 years and about 8 years ago my parents were having issues with their labs, and their vet visits averaging once a week were costing $200-$600 a visit and nothing was happening. One visit to my vet he treated the problem not the symptom and the cost was under $200.
IF you really do like your vet, talk to him or her and see if you can find a way to resolve this without a lot of trauma to the dog and to your pocket. A good vet will work with you and help to find out what is causing this, an ok vet will continue to treat symptoms and not find the cause.
Start with checking for the insects and then you can check the dog food, but if you change too much at once you can possibly make it worse.
If you have any other questions email me privately.