He isn't releasing his tax returns because they don't exist in any meaningful sense, and they won't until the audit is concluded. When you are under audit, the IRS is saying that the documents you submitted are not your tax return, and they are going to use the audit process to create your return.
This is incorrect. The IRS has officially stated that there is nothing preventing Trump from releasing his tax information:
In a statement Friday [February 26, 2016], the IRS said that federal privacy rules prohibit the agency from discussing individual tax matters, but “nothing prevents individuals from sharing their own tax information.”