I think it's great that you're looking to promote the musical and art activity of your little one! As far as art supplies go, here are some supplies that will help to foster his craftiness:
-colored construction paper
Below, I'm including links to arts & crafts and music activities for second graders. To better help you compile a list of art and music supplies, you might want to take a look at some of the activities that interest you, and then view the 'What You Need' list for each.
Basic art supplies such as paper, paints, watercolors, pencils are sufficient for small children. They often love to try finger painting so aprons are good option to keep their clothes neat. You can use large plastic bags with holes cut for arms and head. It is an affordable idea and will increase the interest of the child in reusability of old things. You can also provide scissors, clay, glues etc. To upgrade your listing when children take more interest in art related projects.Similary start with basic and non expensive musical instruments first.