As you realized with your first bf, sex drives are different and his is lower than yours. With bf number 2, the first year you were caught up in NRE. Doesn't matter how much you love one another or find each other attractive, it's impossible to maintain NRE forever. You've had a good go.
That said, one thing that can hit a guy's libido is too much going on. Your guy is working and going to school. That is a big load and a lot of responsibility. Once the NRE subsided and you two began interacting at normal brain chemical levels, the work / school may be overloading his resources to the point he doesn't have much reserve sex.
In other words, probably due to the load and temporary.