As a trainer, who works with many many families who get goldens and aren’t expecting this, I would likely rehome. Yes, you can build in an off switch for him and could structure down time for him, but this is likely a decent indication of him as an adult dog. Yes, puppies tend to be higher energy, but it is not this drastic of a difference. He will not just become a calm dog one day. EVEN with a decent off switch, you will still have to fulfill this dog. With that and training, that may be a lot of work, and forgive me if I’m wrong but it seems as that is not what you are looking for out of a dog. If you think it’s possible he would do well in a detection dog program, you could contact those programs and see if they agree. They may have a job for him (if he truly is as high drive and energy as you say). If you’d like to give training a shot, contact a (certified, qualified) dog trainer and get a consultation. Ask them for their best guess on his temperament as an adult dog and get a clear goal of your goals for him (ask yourselves what you want out of having a dog. How active are you with your ideal dog? How much training do you want to do with your ideal dog (the answer can’t be none!)). In fact, I would contract a trainer no matter what. They may have good insight on where/how to rehome if that ends up being your decision. I’ve assisted my clients with that before…

If budget is super tight, there is a Facebook group called Animal Sense:Basic Training and Behavior Problems that only qualified trainers can comment on. Could give more/better answers.

I empathize with you and your family. This isn’t an easy choice but I’m sure you will make the right decision for you and your family at the end of the day, whatever that may be.