"Bác" if he is older than ur father, and "chú" if he is younger than ur father. :">

Well, just for your information. Thầy is from the words : "Thầy" giáo. People are lazy so they shorten it and just use "Thầy". "Thầy" also have another meaning such as "Father". Some villages in Northern Vietnam still uses them. Thầy means someone who is older than you , or just old and respected by you or others. So that's why it means Father. Giáo means teaching something. Combine them, we have Thầy Giáo means someone who is respected and teaching. It gives the meaning of teacher.

You mean a male in their 30s, but not teacher, right? You can call him as "Bác". It's what i normally use if I guess he's already married or in the 30s or 40s. I don't think using "Anh" is good. Well, it's just my opinion. And yea, "Cô" is for female in their 30s/40s.

i can teach you VietNamese free

Cô, anh, chị, ...