Creatinine Lower and BUN not dropping

[KonaHammond]

Our dog Kona, a 70 lb lab, was recently diagnosed with Lyme Nephritis.  He initially went in for 24 hour IV fluids with a creatinine level of 5.7 and a BUN at 51.  Eventually we discharged him due to costs.  He is still getting an IV during the day and evening subcutaneous fluids.  His values now are at a creatinine of 2.1 and BUN of 115.  Energy and will has increased, but he is still not eating on his own and has diarrhea.  Has anyone ever heard of the creatinine lowering and the BUN remaining high?  Any idea how we can get the BUN down or what may causing it to remain elevated?  Our vet is unsure as well.