When they grow larger, they will produce sideshoots at the point where each main leaf joins the main stem. These will make the plants grow bushy and although there may be more fruit, there may not be time for it to ripen, so most tomato growers pinch out these sideshoots as soon as they are seen. This is known as the 'cordon' growing method. Whatever you do, don't remove shoots that come from the main stem where there isn't a leaf as these are the flowers! Only remove sideshoots for 'cordon' varieties though. With 'bush' types, you need do nothing. Ask your son what variety they are and we can tell you whether to pinch them out or not.
Here's a YouTube video showing how to do it: