How to handle an advisor who regularly gives me new work for my thesis?
I am planning to start my PhD study under his supervision afterwards.


Is he aware of that? If not, you should probably consider what the PhD workload would be, if you are already experiencing the pressure you describe with your master thesis. If he is aware of it, it is possible that he is trying to broaden your views in order to prepare you more for the PhD work and probably being a bit overzealous by doing so.

However, in both cases you shouldn't be afraid to talk to him about where to draw the line for your thesis. That is also an important part of graduation. Personally, I don't take much into account the "mood" of my colleagues when discussing important professional subjects. If you adviser is so unstable, that an objective discussion when he is in one of his moods is not possible, that is another and more serious problem. In that case, you should consider finishing up your masters and going to grad school somewhere else, or at least with another adviser.

As for what you should do, as I said, approach him about your problem and determine where you are with your work and what is the scope of your thesis. It is normal to draw the line somewhere and proclaim that that amount of work will constitute your thesis. Once that is down, you stick to it. As I understand it, you have your problem stated, so you can any other proposed problems delay with "My thesis is my priority right now, but I'll keep that particular in mind, and once I've finished my thesis, we can start working on that one."