Recommender went on sabbatical. What do I do?

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I am an aerospace engineering student applying to several top MS programs for the Fall. I am currently running into an issue with a letter of recommendation and was wondering if I could get some advice on the situation.

Summary:

My professor who I have done all my undergraduate research under has gone on sabbatical this semester. Over the summer he agreed to write a letter of recommendation for me but has not responded to any of my emails since the start of this semester. As the December 1st deadline approaches, I am having less and less faith that he will pull through for me. 

I am left with a few options:

1. Have the graduate student advisor write a letter of recommendation for me. He is a current student in the lab and knows me well, but does not have the status of a professor. Strong letter but less credibility
2. Have a professor who does not know me very well (only took one class with) write me a letter of recommendation. Weak letter but more credibility
3. Wait it out for my advising professor and hope that he submits. Strong letter and strong credibility, but unlikely to submit

I'm sorry for the long-winded explanation, but I was hoping to get a bit of advice as to which route might be most optimal for the application.