Just to reassure you, this isn't a reflection of you doing something wrong - dogs can be scared/nervous without having been mistreated in any way. Different dogs need to be handled differently, and a behaviourist will be able to observe the signals Selby will be giving you and find the ways things that are going to work for you and for him. The fact that he wasn't with you until 15 weeks may have had an impact on things, but he may have had the same issues even if he came to you at 8 weeks.
Honey came to us at 8 weeks, and we worked hard at socialising her, but she is still a very nervous dog - most people who see her wouldn't recognise that as we've been able to work with her to help her manage, and we avoid situations which I know she wouldn't cope with. She shies away from strangers, doesn't trust unfamiliar visitors to the house and new/unfamiliar objects can really stress her out. Learning more about her body language and the signals she was giving us really helped, and we had great advice from people here and at our dog training school...
I have often wondered what would have come of her had she ended up in a different home - we've by no means done everything perfectly for her, and I'm sure we've made mistakes along the way, but I honestly believe that, had she gone to a home where the owners didn't take the time to understand how SHE worked and what she needed, she could well have ended up with serious, ingrained, behaviour issues...
Selby's behaviour is him communicating that he needs something different - you are doing the right thing by trying to find out what that is...
One thing I would add is, at least until you have seen the behaviourist, don't hold onto his bones or take them away from him (even if he brings them to you) - it could be that this heightens his anxiety so that while he doesn't react at that, he transfers the stress onto higher value objects... if someone always kept hold of your dinner plate and occassionally took it away for a while, but brought it back, you might find it a bit annoying but bite your tongue and not say anything if it was someone who was generally very nice to you and who you respected... but if you had had a really bad day and felt stressed out (the state Selby is probably in during grooming) and you got yourself your favourite treat (glass of wine, chocolate bar - whatever would really de-stress you) and that same someone came along to take that off you, you'd probably snap...