When you say he demands respect, what does he mean by that? What is your son doing that is considered "disrespectful"?

Ultimately, what you're going to have to realize is that your home is now your and your DH's home. Your son has no stake in it any longer other than as a house guest or roommate. Your son doesn't get to make the rules. He doesn't get to choose which chores he does or doesn't do. He is an adult and needs to be treated like an adult, with the same expectations that you put on adults who contribute to a household.

Now, that doesn't necessarily mean he pays rent and utilities, especially since he is in college. But he should have a part-time job and should be paying for his own stuff. He should be expected to get good grades. He should be expected to move out and in with roommates by age 20 (I totally understand not paying for a dorm when you don't have to, but he does need to go live with others his own age sooner rather than later). He needs to not be asked to do chores; he should just have a list of things he does without having to be asked.

And your DH's comfort needs to come above your son's. This is your DH's forever home with you. This is your son's halfway house between high school and the real world. You need to be pushing your son to WANT to move out and be part of the world.

You don't have a loyalty bind here. You married your husband. He is the person you have CHOSEN. That doesn't mean it is okay to allow him to be mean to your son, but it does mean you need to work with HIM to make your collective home more comfortable for HIM and YOU. Again, your son is a roommate, not a partner.

What is your son doing that is disrespectful?

Does he pick up after himself and have "regular chores"? Or just randomly does things when you ask? 

The first man was non-interested, and the second seems abusive and controlling.
You may be over-protective of your son, but what kind of "man" denigrates another, demands respect and feels threatend by an 18 yo? You wrap this up in old-school?  Wrong. It is excusing crap behaviour.
Nope, no matter who it is, I will not stand for threats, abuse or a crappy attitude towards my child - and I don't care how old he is. Marrying someone, doesn't mean you are wedded to their opinions or treatment of the people you love - especially your children. I would not choose a husband over my son, when on appearance, it seems your husband has an attitude problem.

sorry but if anyone treated one of my kids like that, he would out before he knew what had happened. I don't buy "old school". There are adult and mature ways of setting rules and expectations. Get rid of that POS.

I can see both sides here - on the one hand, your DH shouldn't be coming in trying to lay down the law and make a man out of your son; on the other hand, the "I'm overprotective because it was just the two of us and his father wasn't involved" screams enmeshment between you and your son, and perhaps you aren't seeing the ways you coddle him and delay his independence, as mothers sometimes do when it's just them and their son together for years.  It's just what BM would say about why SS19 is still so dependent on her (my DH wanted to be involved but was alienated, perhaps your ex really did just neglect his son).

I'd suggest you both sit down with an open mind to each other's point of view and talk this out.

I think you and your husband need a third party therapist to sort out what is going on. 

My ex-husband and my son were like this (he was not his father).  I'm pretty sure this was the beginning of the end of us (there were many other issues)  because I felt a lot of resentment towards him about the constant tension.

Felt like he was being nitpicky ... every. little. thing. was up for argument.

I felt defensive of my son and would swoop and and coddle afterwards.

Looking back now, it was a mistake.  I shouldn't have handled it like that and been more on my husband's level.

Live and learn.  

It is abundantly clear that you married an asshole.  You can change that. Find a real man who does not get his jollies through intimidating his wife and her son.

My mom and stepfather got married when I was 18 and already away at university. He was 'old school' and tried to lay down the law with me. It was awful. I didn't take well to that. I was simply like "Who is this guy?" My mom's remedy to this was to push us more together and DEMAND that I be happy for her and I did not take well to that either. No one gets to tell anyone else how to think or feel. Standards of behaviour can be expected but that's about it. 

Some guys try to establish dominance straight away and think going in hard will make it good. Well, it didn't. And it caused a lot more problems than it solved. I dealt with it by staying very far away. I now live in a completely different country across a big ocean. That was much more about my narc mother than her husband, but the way she handled that relationship didn't help either. 

Funny thing is though - as time went on he was soooo much more respectful of my adulthood than my mother was and we got on fine. 

The generous interpretation of this is that he's feeling an outsider and doesn't really know how to establish a relationship with your son and he's choosing the wrong tactic. The less generous interpretation is that he doesn't want the kid there and he's trying to drive him out. The worst interpretation is that you've picked an abusive dud. 

Counselling can help you sort this out. Hope it's the former rather than the latter. 

I can see both sides here too. Your husband sounds aggressive. Coming into a house with guns blazing is a horrible approach, and telling you and/or your son that he’ll never be a man is abusive. He needs counselling.

At the same time, you shouldn’t be holding your spouse accountable for the damage done by your child’s father, it has nothing to do with him. My DH used to do this to me all the time in our marriage (not about skids). He used to say how bad he had it & blah blah blah. It took a while but I finally realized what he was doing & told him I would no longer be held to ransom for his past with his ex. That was his issue, he chose to marry & stay married to her. He chose to be miserable, not me, and I was done paying for it. It’s not your husbands fault your ex isn’t/wasn’t in your sons life.

I don’t condone his behaviour though. Being ‘alpha’ doesn’t mean being an aggressive abusive ahole who bulldozes everyone to make a point or get his way. If you want to stay with him & make this marriage work, I’d highly suggest counselling for the pair of you.