Provided no one else has ever contributed yes he can. In fact this is not unique to the BSD, you can do that with any license. Even proprietary licenses do not force the author to continue using the same license since they own the original copyright.
How would the enforcement of that work? What if I have cloned the version of the repo that was BSD, and I'm working on a yet-unrelated derivative work when the author changes the license? The answer that seems reasonable to me (not that I expect any correlation between that and the actual law) is that any version of the code that was released with a given license is perpetually under that license.
I'm presuming what you are referring to, and which is likely to confuse a predominantly English speaking audience is that a number of countries with legal systems typically based in Germanic or Napoleonic legal systems, copyright consists of the combination of moral rights on one side and economic rights on the other hand.
Moral rights are generally impossible to forfeit and assign. Economic rights usually (I don't know of any jurisdictions where assigning economic rights to the project maintainer would be impossible - are you?) are possible to assign.
In common law systems, the two are usually traditionally much more closely tied, and when we talk about copyright in English, it is usually the economic rights we're referring to.
While there aren't any software licenses that do that, many projects do it as a condition of accepting contributions. As an example here is GCC's contributor documentation: https://gcc.gnu.org/contribute.html
The short version is assign FSF copyright, or put it in the public domain.
