Hi Rachel,
definitely give him more than just yogurt. There are a few great products out there but I recommend looking at these criteria:
(1) buy products that are kept in the refrigerated section of the supplements department.
(2) buy a product that has both Acidopholus (A.) and Bifidus (B.) strains. These reach different areas of the gut, ensuring that your son's flora will be replenished in both the upper and lower sections of the gut.
(3) Give at least 4 billion parts a day (doctor might recommend a different amount) for a three to four year old...and more if older. Give on a mostly empty stomach 30 mins before eating if possible. Give at a different time from the antibiotic. Give 2 billion twice a day. You may want to start the first few days with 1-2 billion, and work your way up so that he has time to adjust.
I recommend the chewable Flora Bear by ReNew Life. They are 1 billion parts per tablet. I also highly recommend New Chapter's All-Flora...if your son can't take a vegi-cap yet, then you can open the capsule and sprinkle into yogurt or some other medium. I think they are 4 billion per capsule.
I've also had a good experience with Culturelle, but I think that might be only available through practitioners and is a super high count. That was when I was taking it after my son was born and I needed 100 billion a day.
Good luck!