I would agree with JMM...it sounds like because his staining was cleared up for a while, that he has some other allergy...and it is spring afterall.
If you do decide to change foods, I would recommend the NB Sweet Potato & Venison. I did have London on the NB Potato & Duck and 1) She didn't like it very well, but ate it. 2) Her mild tear staining did not improve like I was hoping it would. 3) Her poop was sometimes a little soft (but 90% of the time they looked good). I've had her on the Sweet Potato & Venison for a few weeks now and I think her staining is actually clearing up...so she may have a mild poultry allergy.