Is it acceptable to ask a professor to write a reccomendation letter now to save for later?
It's certainly acceptable to ask. But:


In the US, at least, most universities require the professors to electronically upload their letters directly. So, having him write a letter and hand it to you might not be all that helpful -- you'll still need him to upload it when you apply to grad school. If you're worried about him dying, you might be able to get another professor to upload his letter on his behalf...but that gets a little tricky.
A lot of students have plans to go to grad school N years after graduation -- but at least anecdotally, it seems like the vast majority of those students change their plans and never end up in grad school. So, he may be reluctant to write a letter that has a good chance (at least statistically) of never being used.


I would just discuss this openly with him -- tell him that you plan to ask for a letter in 2 years, and want to make sure that all the details aren't forgotten before then (it is not necessary to suggest that he is senile or at risk of dying). 


He may write some notes to help himself write it, or he may agree to write the entire letter now. 
If not, this conversation gives you an opening to write your notes and e-mail them to him -- including some bullets that summarize your work and others that record any subjective evaluations he's shared with you. Then, two years from now, all you have to do is reply to this message and tell him you need the letter.