If these symptoms started after giving the seed treat, that is the likely cause of what is going on, and not kidney disease. Those seed treats are bad for rabbits because the high amount of sugars/carbs disrupts the natural balance of microflora in the rabbits digestive tract, which can lead to an overgrowth of pathogenic bacteria. This can then lead to diarrhea, which can cause the symptoms of excessive thirst and eating because of the diarrhea causing dehydration, as well as lethargy and weight loss.
If this is when the symptoms started and your rabbit has true diarrhea, then it sounds like this may be what happened. Your rabbit would have bacterial enteritis and this is considered a true emergency in rabbits. And in my opinion based on the information you have provided, your vet doesn't seem to be treating the condition appropriately. Your rabbit doesn't need to be on worming meds or nutritional paste. The appropriate treatment would be sub q or IV fluids, antibiotics like metronidazole and baytril, a sulfa antibiotic if coccidia is involved, questran(cholestyramine) to absorb toxins from the bacteria if it is believed your rabbit could have the clostridium bacteria(which is very dangerous and often fatal), possibly probiotics like bene bac, and possibly pain meds.
For diet, I would be feeding lots of good quality grass hay if the rabbit is still eating well and eats hay really well, possibly temporarily remove pellets, and absolutely no sugary/starchy foods(carrots, fruit, grains, seeds treats, etc). If your rabbit isn't eating well on his own, then syringe feeding a recovery food like Oxbow critical care mix every few hours, would be needed.
I personally would be looking for a different vet. One that is more experienced with rabbits.
http://rabbit.org/vet-listings/
If your rabbit does have true watery or pudding like diarrhea and not just mushy cecotropes, these are descriptions of the possible causes and proper treatment.
http://www.medirabbit.com/EN/GI_diseases/Generalities/Enteritis_en.htm
http://wildpro.twycrosszoo.org/S/00dis/Bacterial/Clostridial_enteritis_rabbits.htm
http://wildpro.twycrosszoo.org/S/00dis/Bacterial/Colibacillosis.htm
This is diarrhea:
This is mushy cecotropes: