Q. What is a good reading goal to challenge a 3rd grader? He is an excellent reader with great comprehension, just lack of motivation.
Be thankful your student already has good reading skills! It's normal for boys of that age to show a lack of motivation or experience a slump -- and he's trying to "fit in" with his friends.
The key is to keep your student reading. That's the main challenge. If motivation is the problem, find a topic or genre that interests him, and feed the interest. (For example: If the topic is "caves," take him on a tour of a real cave, and perhaps even get him in touch with a local spelunking club.) In other words, get him excited about his interests, and he'll want to learn more about it from books. Boys generally like non-fiction books that have lots of facts about subjects like caves, but be sure to include some fiction books as well. There are more than a few adventure stories that involve caves. This will help to challenge his imagination, which will allow him to develop into an even better reader.
At this point, I wouldn't worry about challenging his reading skills. As long as he's actively reading, he won't lose skills. Good luck, and remember, you are a great influence in your student's life. The more he sees you reading, the more he will be influenced to read.