I agree that probiotics are essential to reestablishing healthy gut flora after antibiotics. I prefer powdered products that I can mix with water, yogurt or applesauce, but capsules will work as well. Make sure that you get the ones that require refrigeration. Probiotics are alive and prolonged storage at room temperature will cause them to die off.
Stay away from sugar, caffeine and alcohol until recovered, these all depress the immune system.
If your son will eat them, fermented foods like sauerkraut, kosher dill pickles, kimchi and kefir are loaded with probiotics, and the yogurt you mentioned. Only thing is, you need to find them unpasteurized
. And get plain yogurt without sugar and other preservatives. Don't bother with fancy claims about probios on the uberpackaged, big brand stuff. If you can find something local and organic, or even make your own, that would great!
To further help him recover, if he is not vegetarian, bone broth will be deeply nourishing and healing. http://www.herbcraft.org/broth.html
You can add shitake or oyster mushrooms to this to help boost the immune system.
Eating less and lighter foods in general will allow his body to spend more energy on healing, rather than digesting, particularly if gut flora is compromised.
If he has limited mobility due to the surgery, he'll want to make sure that his bowels are moving regularly. Make sure he is adequately hydrated.
If there is pain, bruising or inflammation at the surgery site, or any kind of emotional stuff like anxiety, restlessness, difficulty sleeping, which are all common with people who are ill and recovering, there are many herbs to address this, but would require more information and would be a lengthier post!
Of course, it goes without saying that rest is a must.
Best wishes for a speedy recovery.