Good question.
Did you ever have him as a teacher? If not, could you/he/she say that you were a student of his? I ask because some schools prefer to have a letter from a professor that actually taught you, rather than one you TA'd for/was the admin of club you were in/takes the same spin class as you.
If not, it may be in your best interest to contact the schools you're interested in and ask if it would be acceptable. Some will say yes, some won't. In general, I don't think it would be too much of an issue so long as it's a science LoR, but it's better if you find out before you send it in than have to scramble to find another one midway through the application cycle.