It is called resource guarding and you need to learn to work with him rather than against him as otherwise it can become a huge issue. I really recommend you get onboard with an good reward based trainer sooner rather than later https://apdt.co.uk/find-a-trainer/
He is telling you what he has is important and the more you try to take things off him the more you are reinforcing that they are important. From his point of view you are being aggressive trying to take it off him and he is defending his treasure.
If you really need to take things off him then you need to swap for something high value so he is happy to swap. You also need to evaluate what you need to take off him and what you don't and stones are something pretty much all pups will pick up but the only ones who persist are the ones where the owners consistently try to stop them by taking them off them - every time you do that you are raising the importance of stones and increasing the likelihood of him picking them up.