I have a male YB who stopped eating back in November as well, he lost just over 100g. I actually pulled him from breeding in February after not eating for 3 months. I tried regular( for him) sized rats, weaners, mice and gerbils with no luck. I finally gave in and gave him a young ASF this last Tuesday, it was gone when I checked 10 mins later! It was small for him but I plan on trying a ASF scented weaner rat this Tuesday and hopefully get him back on schedule, he has his own girl this year after all, no sharing!
Although as long as he doesn't look skinny try not to worry(ya, right). My Loki is looking very lean but not skinny. Oh, he doesn't look bad at all.