My husband's work is taking advantage of him

At first, this job was a blessing to our family. Now, after 2 years of working there it's so obvious they're taking advantage of him. He's a tech manager and works full-time and more. He only makes $18 an hour while all the other managers make salary and work wayyyyy less hours than he does. He's been busting his ass. Every 2 weeks he has to work on-call shifts for 4-6 hours, even on his days off. This is required and not optional and it's late hours after 9pm. Lately they've been forcing him to work for hours on his actual days off. We have a daughter and another baby on the way and they're still forcing him into all this overtime while barely paying him anything! We hardly get to spend time together. His boss is always saying "you're getting promoted next, I have big plans for you" but just recently his coworker (who is also a manager) was promoted again. And he already makes salary! He has the same exact job as my husband but does less. He never works on his days off. My husband is a hard worker. He's too nice and will always help out whenever he is needed. He's too afraid to bring any of this up because he doesn't want to get fired but I think its so wrong. I'm furious that his time is being stolen away from us while it's not even worthwhile because his paychecks are so small. I'm just so upset and he is too but I have no way to help him if he won't talk to them about it. And they keep laying these big projects on him that are taking up even more of his time. What should he do?