Let me start by saying that you are the one who has to make the decision, and you are the one who knows your dog best. I can only give you input based on a quick internet description. We had to euthanize our Aussie mix because he had degenerative epilepsy that was causing aggressive outbursts and attacks. He was a loving and well behaved dog when he wasn't having seizure activity, and it was the most heartbreaking thing I've had to do with a pet. Before we found out the cause though, we had people telling us that he thought he was dominant, that we needed to be alpha, that he was bored, that he needed a treadmill, that he needed a raw food diet, and so on. We knew this wasn't accurate for our dog, and it just made the whole process more painful. So, bottom line is, you have to do what you feel is right in the end.
The sudden behavior change is what I would focus on when speaking with a vet. To me, it does sound like he was used to paling around with mom, and is bored and stressed as a result. I would definitely try things like hiking or playing energy intensive games with him for a few weeks. If that doesn't have any effect at all, I think you would be better equipped to make a decision from there, as you would have more insight as to whether that seems to be the issue, a related issue, or not the issue at all. If he was showing aggression at all, my answer would be very different. But, it sounds like you have at least a bit of time (if making a pen in the mean time) to explore that route or any others that seem like they might apply to your situation. I would also be careful to not leave him alone in the pen for long ammounts of time, as that is likely to worsen the situation. Best of luck to you.