You will not need to purchase items to get started. Encourage your child to follow projects for several years. The fun and work atitudes he develops at his science hobby will last him a lifetime. This site and its many links help you find ways to help your child progress with his science interests. Success is the main motivation in life.
People enjoy things they are successful at.
You will foster your child's interest in science and his other studies by helping him succeed with these projects. A little help from you when he is blocked by a problem can enable him to continue. I succeeded until I came to Ferric and Ferrous compounds midway in the book. The difference is simple, but I failed to see the difference and concluded I had reached my limit and quit. It did not occur to me to ask a teacher or write a postcard to the town doctor.