Lyme disease doesn't "come and go" however if it isn't treated, the symptoms can be similar to what your dog is experiencing and it effects the kidneys. Kidney probs can cause dehydration. Could also be pancreatitis, addison's disease and more. In my opinion, I would demand the vet to be much more assertive in his testing - and/or have a second opinion. Many things can be undiagnosed simply because the vet office does not have the proper equipment.
So many things this could be. This constant rehydration is only managing the problem, not treating it. I'm sorry I couldn't help you directly. I'm concerned about your dog and I hope you continue to search what is going on! He's so young I hate to think of him having to go through this all the time.
Wish you the best
Please reply to any questions or let me know if you need further assistance!Otherwise, I hope you have found this answer satsifactory and worth acceptance!
Oliver's Mom