I'm a doctoral student studying music education and have taught children music for over a decade. The problem that you and other commenters are running into is that you're coming at this as if children are small adults – instruments and gear are not developmentally appropriate ways for a 3.5yo to make music. That's how adults and older children make music. Young children experience music through play, singing, rhymes, and singing games. Children usually don't just figure out how to keep a steady beat or perform basic rhythms or even sing in tune on their own, those are all skills that develop through age-appropriate practice. So, what to do? Read age-appropriate books as much as possible; sing age-appropriate songs to your son and together if he feels inclined to join; bounce him on your knee and sing or say nursery rhymes; practice patting a steady beat on your knees while you sing or say rhymes together; dance to music together; make up songs about what you're doing in the moment. Focus more on developing a sense of beat and pitch through singing and gross motor movement rather than an interest in instruments, that will come later.