I have had parkinsons for 10yrs fatigue an issue, exercise helps me once I push myself to do it! Exercise releases those feel good chemicals as well.
Keeping active is important have a look for group sessions may increase motivation to attend.
Tiredness seems worse after resting too much we can get sluggish when we really need activity. Exercise often helps us to sleep better too.
It maybe , he needs a bit of help with low mood and it is possible anti depressants on a short term basis to ‘kick start’ him into action may help.
GP appt , your own doctor could assess him for depression, or refer for counselling if pd getting him down ?
There is PDUK helpline, keep your husband updated and involve him as feeling in control is often important to people with PD.
Take Care and I hope you both feel much happier soon. Xx