I don't think it is unreasonable at all to not want a grown man in your house while your husband is gone. 

The problem is your husband doesn't see him as a grown man. At 26, that is what he is. 

This would be a hill to die on for me. If he isn't disabled and has the ability to get a job and get out of the house, he needs to do that. 

Your husband thinks you are overreacting because you don't want his 26 year old lay about son, laying about your home? Especially, while your husband is gone?

Tell your husband to take his loser son with him, then.

You have been putting up with this for quite awhile, now. You are going to have to figure out what you really want to do, and it might mean making a hard choice.

Doesn't matter if you're being unreasonable or not (and you're not). It's your home, and you expect safety and security and comfort. DH should be turning himself inside out to make sure you have that, especially while he's away.

But it's harder on HIM to have to tell his son to find someplace else to stay, so he's going to make it your fault in hopes that you'll back down.

Son goes with Dad, or son couch surfs with a friend, or, how about this, son gets a job and his own place to live.

Even if he is admitted to a PhD program, that won't start until fall. And applications would have to be submitted by now, for the most part, or at least with any hopes of getting a paid fellowship. He needs a new launch plan.

There is nothing unreasonable about expecting an adult to actually adult. 

DH is the the one with the problem for enabling his son and encouraging him to be a failure in life. 

This is just idle curiosity but ... what kind of career opens up with a PhD in philosophy?

Btw he has a degree in mechanical engineering, had a great job worked for one year then quit to go back to school. 

So, about this ^^^^^

Was this a discussion between the three of you? Did he ask if it was okay to move back in or did he just quit his job and land on your doorstep?

I am all for getting more education, but at his age and having had a great job, seems the onus should have been on him to either have enough money in savings to continue to support himself or continue working while going to school. And, he quit his job before even knowing if he got in?

I just would not stand for this, especially with his attitude that you described in your forum post last year. Three rejections? Time to get a job and figure something else out, and not at your expense. He needs to go.