Hi welcome to HCm 
I have gone thru your query regarding rehab exercises which help one get fit after 
appendectomy . It  is indeed a good sign that your son  is back to eating well and on track for a full recovery  . 
Dear , for your info , our body works well ,till it is not broken and repaired .
A repaired part , always gives pains if you over exert , can 't tolerate jerks of increased amount of intense exercise  and other problems . 
As he has recovered , he should not worry at all  .  He will  be able to achieve his goal  gradually .
Always start  from warming up exercises and gradually increase stamina 
The strong immune system , which is helpful for our disease free total health , is incredibly complex because your body has many lines of defense to rid itself of foreign invaders.  There is no better option than physical exercise. Brisk walking is the best exercise to begin with.
Yoga -Pranayam - Deep Breathing , Kpalbhatti Rolling , Streching exercises not only help you gain  strong muscles strong immune system and security from diseases but also gives you well shaped body
Modification in your life style , food habits -include fiber,essential nutrients & 
antioxidants in natural form  ,cheeze, milk , butter , veges ,fruit  , with required work out , Yoga , pranayam deep breathing , are the piller of our strong immune system responsible for healthy disease free life .
To give resistance from diseases & alley pains take a level spoon of turmeric powder with hot Milk + a spoon of almond oil , at bed time . 
Lemon juice with water , a pinch of salt and pepper before meals .
Aloe vera juice + Amla .
Ginger , garlic  omega 3 - fish ,flax seeds ,coconut water  , nuts all  help strengthen  the body and mind .
It is suggested to consult your doctor for proper guidance if problem persist
Hope this helps solves your query .Take care .All the best .Get well soon
Don't hesitate to come back in case of any further query .