Great that he's using symbols at school. Check what kind they are (most likely to be either PCS or widget), and stick to the same. If they're using PCS, then I honestly would avoid the iPad altogether.
Also, just be aware that within each symbol set there may be several options for the same word. And schools often customise them a bit too. So once you've decided which words you're going to target, it's worth just checking with school that they're using the same.
Symbols always need to be taught explicitly (unless a child can read). Even if you think they look obvious, they're really not. Most able adults (me included!) would struggle to look at a page of unlabelled symbols and know what they were... Really often when AAC systems don't work, I think it's because people have missed the step of actually teaching the symbols.
I think you're right as well to aim to get something really simple and motivating working. Try and tie it to some really functional activities during the day, and make it powerful for him! If it starts to work, then yes you have a powerful argument to take back to school and ask for more specialist advice.