You should have a sit down and tell him about how you are feeling.
If your boyfriend is always falling asleep in the middle of talking to you because he is tired, a 40 hour plus workweek is not a good enough excuse.
It is his responsibility to get the appropriate sleep necessary each day in order to keep him alert.
There is a good chance that your boyfriend may be spending the time in which he should be sleeping doing other unnecessary activities.
When you have a sit down with him, find out what he does when he is not at work.
If he is spending too much time playing video games, surfing through his social media or hanging with friends, these are areas that he should make adjustments to.
It is all about time management.
If he makes a conscientious effort to manage his time better, he will not be so tired whenever he talks to you to the point where he falls asleep in the middle of conversation.