Children like to do things,
1. Give him small tasks to do, like making models of the solar system, planets, space ships. rockets etc. monthly one task on weekends.
2. Get him to read books on space, solar systems, space ships, etc.
3. Allow him to watch movies on space, star trek, children's movies.
4. Make time to collect the names of stars, planets, zodiac signs, and other solar bodies.
5. Make him write essays or short articles on satellites, names of satellites, which countries launched which satellites, etc.
6. Make him understand the parts of rockets, provide reusable things/waste things to make the rocket model. Make him learn the parts and it's working.
7. Search for space quizzes and tell him to participate.
8. The child can make a table of the list, about the data of all the planets, its rotation, revolution, mass, size, etc.
9. Tell him to read newspapers and make him collect information on space and then make a book.
10. Collect information on space scientists and about the space travel the requirements, the environs of space ships, etc.
These are some of the simple and interesting things that can be provided for the child for self-learning and to understand about exploring the space topics. Introduce the child with this one at a time, review, and see how he is learning/grasping and then move on. Choose the simplest one first and then to more complex as the child learns. Do not burden the child with too many learning things, as the child has to focus on school works, tests, projects, and other activities.