The thing is he doesn't even have the energy to THINK about doing cardio... much less hopping on a bike or going to the swimming pool... you just can't know what it's like if you've not experienced it yourself.
Stopping Kratom can be a mess... I was there... basically went from 40+g/day to nothing cold turkey... had to take a full week of vacations the first time I quitted for I was both physically AND mentally deprived of every single bit of life in me... just couldn't leave my bed no matter what... and trust me I tried...
Took advantage of that week to stop ALL stimulants/alkaloids... not even an espresso or cup of green tea... full detox mode.
After about 10 days I was feeling like a charm again... like prior I even touched the stuff for the first time... you'll make it, Simon, just accept that you'll be in hell for 1-2 weeks, then it will pass... like everything.
Good luck bro, I know how you feel, believe me.