Does he have office hours? I'd suggest going to those and asking face-to-face. This is the best way I've found of asking lecturers to write references, especially if they are on good-terms with you. I'd also think carefully about whether you want this person to be your reference - if they're known for not replying to emails, consider whether they would send off your reference by the deadline, as you wouldn't want them to jeopardize your application. Of course the person who supervised you is very well positioned to write your reference (and it is the norm for a previous supervisor to be your reference), but this isn't helpful if they're unrelabile with deadlines.
I had a similar issue whilst applying for my PhD, and eventually had to email another academic in the department stating that I had contacted my first-choice of reference as he had worked with me more closely, but had not recieved a response, and therefore asked this other lecturer to write me a reference. The other lecturer agreed to write my reference, but asked if he could chase up the original reference for me as he agreed that this person would be better suited to write my reference. The second person I contacted did end up being my reference (as the first did not respond until after the reference deadline), and even though he didn't know me as well as the first, I still got the funding so do not believe this significantly impacted my application.