From the PET it seems like he will be placed on some form of ADT - androgen deprivation therapy - to slow down any loose cells from growing.
Radiation may also be considered - over the coming months after healing or on a wait and see basis. If so, it is VERY important for your husband to maintain a vigorous schedule of exercise and weight training. Can’t emphasize this enough!!
Since you mention that you yourself work out, encourage your husband to do the same - maybe even do it together. My wife and I can usually be found together - she on her elliptical, me on my treadmill. Or run or cycle outside together, whatever you desire.
At first, my wife often chided me for exercising so much - especially when I complained of fatigue and pushed myself to get through it….she thought she was helping me. But now she’s come to see the importance of maintaining muscle tone and weight when you are on ADT and she encourages me to exercise but not kill myself doing it.
Best to you and your man - you both got this!!