How to handle an advisor who regularly gives me new work for my thesis?

I'm currently working on my master thesis and will have to hand it in in April. Of course, the topic of my thesis is somehow restricted to a certain problem, but my advisor keeps telling me new problems to consider for my thesis, which have little relation to my original master thesis problem.

The problems he gives to me are very interesing, but they do not fit into my master thesis and if I had to include them, I would get into serious time troubles. I talked with him about this and he always answers very vaguely, e.g. with "Let's see how your thesis turns out to be". His answers are also very mood-dependent, so one has to be careful.

How can I convince him that his problems are not fitting for my thesis and that it is simply too much? I have to hand my thesis in soon and now, at this point, he comes up with new stuff to include. Furthermore, I am afraid of getting a bad grade if I don't do what he tells me to do. I am planning to start my PhD study under his supervision afterwards. Could I perhaps propose that I work on his problems after my master thesis?

Thanks in advance, I appreciate any sort of advice.