My first husband was a pre-med student and had much the same schedule as your husband has with school, studying and work. I don't know if he cheated on me -- I guess it's possible given the amount of time he had apart from me on a daily basis -- but I never had any solid reason to believe that he was unfaithful. One of the things that drove our marriage apart, besides his basic overall immaturity, was how low I was on the totem pole of his life. I was basically the only one involved in our relationship and I wonder if that is what you are experiencing with your husband now.
Now I know that when you work and go to school, you just don't have a lot of free time but I wonder if he has lost his ability to balance it all with his family life as well. If so, I think this is probably very common. I would talk to him about what you have been noticing, what you need from him and see if the two of you can work out a better schedule for the two of you. Maybe some couples counseling may help. But when you talk to him, make sure that you are doing it in a calm, understanding way. I've noticed that men can get so defensive so easily if they think that we are accusing them of failing us in some way.
Hope this helps put a different perspective on your relationship. Wishing you all the best.