EE here - he technically has to comply with FCC Part B since he's selling in the US. That said, the hobbyist keyboard community has been pulling this for a while for small group orders without issues. Pre-certified modules are helpful, but stuff gets weird when it's on a board, so no guarantees. The fact that it's on a single layer board doesn't instill confidence since that typically means less than ideal return paths that can cause problems.
In the EU it's illegal to sell without external certification if the product is a medical device, is made for children or contains a radio.
Certification will typically cost you $5,000-$15,000 depending on which lab you use.