Ehhh.....hes only 20...not exactly a grown man as far as Im concerned
Honestly, thats a tough one because you're both very young and only 4 years apart. People will look at the "legal age barrier' as a ruler which doesn't make a whole lot of sense since, in 2 years, you will be 18 and 22. Perfectly acceptable? Why? You're not going to be much different in 2 years than you are now. You'll both be in School - granted he will be getting ready to graduate.
Since your living in their house, as long as your parents approve of him I say its fine. If he were any older i would say no of course but its right on the edge as far as im concerned. Just my stupid opinion