I don't quite understand this game is almost over, or the dev is doing the other game in parallel, like if he ends now he would think it's dirty we haven't even been explained about the elf princess story and we don't have an ending yet for the dev to finish. and in this new game it will be a female protagonist as well or it will be a man.
You will probably get a better answer, but i'll write this anyway based on what i've read so far:
The dev will finish this game first. If i remember correctly he said that the story is about half way through, but i'm not sure on this part.
He is planning to make multiple games in the same world he created. The next game will be centered around the sister of the current MC.
He is indeed developing the next game already, but only in the sense of planning and designing so far. By doing so he can ask for input and run polls while he is working on the current game.
Judging by his work speed i think he might save a few weeks worth of preparation time that would otherwise go between the end of this game and the release of the next one. Quite a smart move.
So no, i haven't seen anything so far that would indicate that the dev is focusing on anything else than this project. Right now, there does not seem to be a reason to worry.