Ok, to start let me qualify my answers first by saying that I am not an expert on the full suite of medical benefits to be found. My experience with CBD is with people who have neuropathy, sleeping problems, and especially seizure disorders. Also it's very good for people with anxiety disorders.
I'll have to search for case studies, but please understand that due to the nature of the plant's legal status, those studies are very few and far between and the vast majority are focused on showing the harm done by cannabis, not its possibilities.
I wouldn't even begin to try to suggest dosing, but given my experience with CBD I believe it would be exceedingly difficult to give him too much CBD. This is not true of THC, which I feel is probably another important cannabinoid and IIRC has been shown to drop blood sugar levels (the cause of getting the munchies). I would start with the smallest amount to purchase, use it and see if it's even helpful for him before going for more, that's just how I am, I want to sample before I buy.
This link seems to confirm my understanding of the benefits of CBD.
http://www.canorml.org/cbd.html
http://www.medicaljane.com/2012/12/20/cannabidiol-cbd-medicine-of-the-future/