I was discussing about this with someone on this wiki, about this topic, and that time I thought that this point has not been explicitly taken in Vol. 16. Some might say that he is still a guardian as nothing on the contrary has been said, while some might say that after the announcement of him being Maya's son & Miyuki's fiance, nothing has been said that he would no longer be guardian and be replaced by Minami.
I would put my own views after there are a few comments in this discussion. So, guys with LNs upto Vol. 16 & the spoilers and summaries for the later volumes, let's see what opinion people have about this and their arguments for and against this are.