D.B.
I think someone else should look at his resume, not just you. Someone objective. And he might listen to that person too.
Most jobs are not found in want ads. He should network (which I realize is hard for an introvert) but that's where connections are made. BNI, Chamber of Commerce, etc. He might consider contacting some head hunters. Since you seem to be very strong in feeling he's completely barking up the wrong tree, he needs to look to someone else. Let him send resumes for anything and try to help him categorize them (sales, mechanical....) and see which ones he hears from and which he doesn't. If you can establish a statistical pattern (just general, not tons of math!) of who calls him and who doesn't, what he gets interviews for and what he doesn't, it will be more helpful. Going to large employers with a ton of openings can sometimes be efficient - if he can get an interview with an HR person who interviews lots of people, that person might remember him when a different opening comes up.
Meantime you can get some common interview questions from an HR professional (and there are plenty of freelance people who help do this, work on resumes, etc.) and let your husband start answering them as practice. If all his answers are "I'm a hard worker" and "I like people", he needs better skills to really stand out.
Telling him he needs to go back to school might just demoralize him, and telling him he can't choose the best ads is going to beat him down even more than he already is from his lousy job. Remember he is vulnerable now. If you have some insurance that covers short term counseling, believe it or not, that can help him set goals and identify his strong suits. I'd try to push him in the direction that will yield a change in his behavior, rather than try to get him to change on his own.