My experience with my boy is he's gone through days with low appetite. He is also 9 months old and actually just went through a day of not eating his food at all and a couple days of not eating very much about 2 weeks ago. He would eat treats though and I'll give him food as treats too when we're training.
I stick to the instructions a few trainers have given me to not leave him his food plate for more than 30 minutes but still give him food 2x a day - I've been told it helps make picky eaters learn they have to eat when served.
When he's gone underweight, I add a few treats or a scoop or two of canned food or toppers. If there's any sign of digestive upset I've mixed in a scoop of organic nonfat plain yogurt or pumpkin puree - whichever I had at hand, and he loves both so he gobbles his food then.
Good luck with your boy. Was he otherwise ok with the BB or did this start when you switched his food?