Many possession problems can be worked through.
The first and obvious tip is NOT to attempt to just snatch items from him and turn it into a battle of wills, as he will just feel even more threatened.
It depends on how aggressive he becomes, but for many dogs, the best initial step is to offer them something MUCH more interesting than the toy or bone they already have, eg chicken breast or other meat, and gradually build up to making an exchange. For eg, begin with just offering some tasty meat to him without trying to touch his prized possession, until he starts to realise that other people approaching him while he has his favourite things are not a threat, they are offering great things to him! Eventually you may be able to build up to slowly moving away the toy/bone as you give him the treat, and praising him if he allows you to. Then give the toy back with more treats.
Another tip is not to allow him to have his toys and bones at all times. Keep them to yourself, so they are YOUR toys and you choose when he has them. Bring them out when you choose to.
It's all baby steps though...it would be a big mistake to allow him near children or other animals when he has toys or bones, only bring them out at safe times. Gradually he will learn to associate sharing his things with rewards.
It would of course help to try a behaviourist too