My husband does fieldwork and has to travel during the week and only be home on weekends for up to 3 months at a time every couple of years, so I understand. It will be fine once his class is over and things settle down. In the meantime, ways to help are:
video-cam if possible - if you can set one up at work and he can talk to your son on a 10 minute break or at the end of his work day, like around dinner time or something. Seeing his dad everyday even briefly will help with the connection.
Talking on the phone - I would put my husband on speaker phone while we were eating dinner every day. Maybe while your husband is diving from work to class he could "join" you for dinner? Or bath?
Keep yourself busy and him busy. I would go eat dinner at a friend's house once a week and plan lots of late afternoon playdates. Keep him busy during that time of day when your husband usually comes home . The socializing with other mommies will also help you immensely.
Also though I assume as you work on the weekend that your child sees your husband lots, have them do something "special" together on the weekends. They could go to a park, on a nature hike, a sporting event, etc... And make a "date" one night a week with your husband. You don't have to go anywhere, just spend two hours with no TV, no computers, just you and he where you can talk and reconnect.
It is ok to tell your husband it is rough with him gone, just also tell him you support him and what he is doing and you are willing to sacrifice in the short-term for the long-term gain. Then tell him ways to help "ease" the bad times. Better than yelling at him in a few weeks when it becomes unbearable. Believe me, I've been there. You will get through it.