@ drift
could you maybe make a video where he trys to get down? there you really can see what is wrong.
and yes there young dogs who can be in pain from hurting joints ... also puppy who are so badly in muscle or bone or breeding, or livig in small cages when young or such ...or have hurting hip, leg, neck, back, paw or belly that they really cant or want to lie down, it could hurt to get down, or stand up after, some dogs only show little signs of it that you dont see when you dont know what to look for...it is sadly not only old dogs who have little or big pains, or he did grow to fast and is hurting a little, ..some a chiro can help you with, other pain meds or the right sport, massage.
maybe one can ask the old owner, if he did show it with them too....maybe it is really only playful and bad teaching..or a game...voice and you Body movement can say to him lets play
when he really is healthy and you checked the whole body for pain (light touches and pushes) ...then maybe try not to talk when you want him down, stay silent, back straight shoulder back and give him the sign to do it, then he will not bark and maybe goes down when he was trained with sign for this, maybe... and maybe use the moment when he goes down to lie next to you or his bed to say down....and throw treat when he does it.