First of all. . . men's pants are sized by measurements. His waist is the first measurement, his inseam is the second. Example: 28 X 30, 30 X 34, etc. Being that your boyfriend is the same height as myself, then his inseam is most likely 30 or 32 (that is the second measurement). It's a matter of knowing what his waist's measurement is.
If you don't have a fabric measuring tape (like a tailor or seamstress uses) then you can use some string, then simple use a ruler to measure the string.