You can't force him to go to school, especially at this point right now in the economy where having advanced degrees isn't helping a lot of people right now. Many state jobs require Masters degree OR 5+ years experience. Your husband has 10, so in the state's eyes, it's roughly the equivalent of a Masters. I know this b/c my husband also has a job with the state (that also pays very little) and I have read too many job applications, pay scales and requirements that say this.
It sounds to me, that education isn't necessarily the answer for him, but perhaps a change in careers or at least a change in employers. With his years of experience he could shift into a new field, perhaps private sector in which he can apply some skills.
Also, we are in our 30s, make less than $45k a year and are having a very hard time financially. I stay at home because I can't find a job that would earn enough to make paying daycare for 2 kids worth it. I work on the weekends and it helps, but that's all we can do. So, be very grateful that you even have what you do. If two full times jobs aren't enough for your family, then perhaps you need to cut back on spending on some areas.
Really, you have an attitude of what's yours and his. If you change your focus on the two of you contributing to your family together it would be better for your marraige. What we do, is funnel our money for living expenses into one account. Then, any extra we budget and spend between us and out family. Having a clear budget could help you guys a lot. I'm fairly certain he is aware he makes less than you. SO if you change the battle of the money issue, and instead attack it from the issue of him finding a job that he will be happier at, it could help. It sounds like he could be stressed and depressed about this, and you pushing him and pretty much belittling his ability to contribute to the family is a major stressor.
It really sucks he doesn't earn more money, but guess what, PLENTY of people are struggling right now, and employers just aren't giving raises. He needs less nagging about getting a masters, and more encouragement to try his hand at a new job. He isn't happy where he is, and he isn't making much money as well. He could get some free career counseling at my church, non-members are welcome with no pressure to join. They help people learn how to market their skills, help with brushing up on resumes, dossiers, interview skills, networking and getting back into the job hunt and tons of job search tips. It really sounds like this would help him considerably. You can find a career center near you here:
https://www.ldsjobs.org/ers/ct/stakes---wards.jsf?name=th...