My dog is usually very playful, but I recently took him to the vets and apparently he has a stomach infection. He is not his usual bouncy self and I am very worried?

What course of action did the vet take for the infection?

If he gave you antibiotics, how far into the course is he? (how long has he been taking them)

Once you have finished the course of antibiotics feed some plain, natural yoghurt daily. Just mix a tablespoon in with his food or feed it on it's own if he will eat it.
When a dog (or human) has been taking antibiotics it kills ALL bacteria, including the good ones. Yoghurt contains good bacteria and can help bring the balance back to your dogs stomach faster.