I think the first question you have to ask yourself is do you want to keep the dog? If the answer is no, then explain to the previous owner it hasn't worked out. If she doesn't want him back, take him to Dogs Trust and don't read on.
If you want to keep him, then I would advise a behaviourist. Not a trainer. If you ring your vet they should have the number of a behaviourist.
Without seeing him it's difficult to 'diagnose' the problem. But it may well be fear aggression. So in the meantime, keep children away from him and allow him to settle in. People tend to humanise dogs. For example, 'He is not allowed on the bed and I tell him so'. If you think about it from the dog's point of view, where he has probably been allowed to do this for 6 years, he's totally confused. If you stand facing him, possibly hands on hips and sternly reprimanding him, then to a dog, you are 'blocking' him and looking like a threat for his 'safe' place. He doesn't know, how can he, and he doesn't trust you yet and in his mind you are being confrontational. Carry some treats about your person and if he jumps up there, turn your back, move slightly to the side, don't block his route, point to the floor, say 'off' in a no nonsense voice. If he jumps off immediately toss him a treat. If he doesn't, toss a treat across the room and say 'good boy' when he gets down. It may take some patience. Don't grab him and don't 'block' him. Dogs follow gestures far more than voice commands because it's what they use amongst themselves. It's a miracle they can figure out what us humans want them to do if you think about our gestures and windmill arms when we're trying to get them to do something. Again, if he has picked something up that he considers high value, then trying to get it off him isn't wise. It's his, it was on the floor, why should he give it to a total stranger. He has probably not been trained as a puppy to surrender his 'stuff' as the consequences are good. A tasty reward. This will take some training and can't be explained well without demonstrating what to do. But he must learn to give it up and there will be a good reason to do so. A nicer toy or treat. Dogs learn by repetition and learned behaviour, not because they 'just should know'. If you want to keep the little chap, keep your grandchild safe. From his point of view right now, he's been plucked out of his home, lost his owner, taken to a strange place full of strange people who tell him off and want to take things away from him.
As someone said previously, he didn't make contact even though he could have. He was just giving out a warning. He is probably not used to children either. You must not, under any circumstances, give him an opportunity to bite. Good luck with him.