I'm a master student in statistics. I took this professor's class, in which I've done very well. I have discussed with him about my research project a few times while taking the class, although he is not my thesis supervisor. It's been 2,3 months since my last time meeting him. I shared my paper draft (written over this summer) with him recently via email since he seemed genuinely interested in my progress and even suggested I should keep him posted. I'd also like to ask him for a reference in the fall for PhD application. The draft was quite lengthy although I think the content is at least original and interesting for a researcher in my field. So my guess is that he doesn't have time to go over it and does not care enough to meet me for a discussion. There is a possibility that he might have missed the email. There is another complication that he suggested one time if I wanted him to supervise my research but I have already chosen another professor as my supervisor at that point. So he could have felt uninterested after knowing that, although we had one more discussion after that incident.
Question: How and when should I follow up with another email? I really want his reference but do not want to appear like a burden (that's why I did not reach out to him for the whole summer until now). I may be able to go to his office uninvited but I'm not sure that'd be a good strategy.