Hi! At 8 months he is probably trying to test the boundaries a bit but you‘re obviously right to address is early. First and foremost, do try and stay calm, its common to react by getting cross but raised voices or scolding tend to turn it into a conflict situation. When you tell him to go to his bed, does he see his bed as a place he loves to be in or somewhere he has to go to? (for whatever reason). When he‘s lying in his bed quietly, drop a treat in and praise him when you go past and make sure hes not disturbed by anyone else when he‘s there. Never use his bed as a punishment.
Another problem could be he‘s overtired, although he‘s out of the puppy stage, he still needs plenty of sleep and tiredness can often cause them to be uncooperative. Setting up a routine might help - for instance (e.g. you could try making the evening feed the watershed for no more action, let him just wind down and eventually he‘ll learn that there are playtimes and times when there‘s nothing going on so he might aswell go to bed!
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You can start training him to give up what he has in his mouth by offering him a high value treat as a swap. Start training this when he‘s alert and happy though so that he learns that giving up whatever will earn him praise and reward and it becomes a natural thing to do instead of a power struggle. I made the mistake of forcing things out of Humphrey‘s mouth and its resulted in him hanging onto everything for dear life, so we‘re in the process of retraining this at the moment! I‘ve also found that confronting him head on makes it worse (staring into their eyes is bad manners in the doggy world and is interpreted as aggression) so looking to the side or even positioning yourself on the side of the dog can help.
I think the key is staying consistent but above all reward good behaviour and totally ignore the bad so that the dog learns there is no advantage to him behaving that way. You could also try distracting him with letting him do something he can do already (sit + praise + reward).
They‘re sensitive little souls and respond much much better to praise than scolding.
, - I‘m no expert and only speak from what I‘ve picked up along the way and what has worked for me but hope that helps!
Best of luck!