the baking soda should not be bleaching his hair at all. it should not have stayed in his hair. did you rinse the hair thoroughly with water after the cleanse? If the hair was not rinsed, that could be the reason there are white patches, literally being the baking soda still in his hair.
Also, I have heard a few people have issues with white gunk in their hair. It was always caused either by wax, or by some shampoo with residues in it, however. To get the wax out, many people use dawn dish soap. I know it sounds bad, but it will not harm your hair, and is one of the ONLY things that can actually get the job done. It will require using about a full bottle of dawn, mixed with hot water,and scrubbing on individual dreads. If you search the site, you would be sure to find very specific instructions. If you are serious about doing this and have any questions with that, I can get you better instructions.
Also, the shampoo is sometimes the culprit for 'white gunk'. Make absolutely certain his shampoo is a residue free, 'dread-friendly' shampoo. If not, it will leave many different moisturizers/conditioners that will remain stuck in the dread, and build up over time.
A picture might help other members identify the issue more clearly as well. These are the best theories I can come up with for the time being. I personally do a BS rinse and a ACV rinse, and it works wonderfully for cleaning my hair. Good Luck.. Keep us posted