Well, I am not the FSF, but once again: You do not get to decide that.

Once you published Boom (which you did for the TestNet already), you have to publish the source too. The GPLv3 license requires it unconditionally.

And not when you decide to do it. Your mode of operation basically is similar to BHD (where the authors also decided to "open the source once the system is stable"). Which means exactly 2-3 things:

• You suggest your code is not stable.
• You start out your coin in an illegal way. Please be aware that everyone running a node is subject to ISP take down notices and license violation lawsuits. Now this may not bother you(?) in some anonymous way and behind some great firewall of China, but if you want people to run the nodes in sane jurisdictions, it becomes a problem.
• It may hint to a security by obscurity concept. You believe that you gain security for the coin by obstructing access to the code. In a business such as cryptocurrencies, where strong cryptography and proven mathematical concepts are key, obfuscation is a very weak protection.