I think you need to have a talk with your husband immediately. Marriage counseling may be in the future. Depends on what you and he want out your marriage.
I do know, It is not normal for a 30 year-old, married man, with children to want to hang out with a man who is 10 years his junior, and probably has the mind of a teen ager.
Do you think it is normal that he is choosing this kid over you and your children for everything? I mean, invite the guy over to go shopping at Target with you and the kids? Weird! Spending all of his free time at the kid's house? Weird! Do you really think they're playing video games down there all night? Normal adult men, who are in love with their wives, and adore their children WANT to spend their free time with them, being a family. Not to mention, by the time a man is 30, most are psychologically well past the need to be a kid and throw responsibility out the window.
I think it is really Freudian that you say in the first line..."here's the low down on the situation. (Men who cheat with men are considered on the "down-low") ..." and titled the post " My husband is having a bromance." I think it is very possible your husband IS gay, and as long as you allow him to indulge with his affair, the children and you'll be the third wheel.
He is not having guy time with this kid AND a group of friends for some male bonding like he probably did in the guard. That isn't guard/military culture...sorry. Too much machismo for one-on-one stuff and trips to the Target boutique. It sounds like they aren't meeting up with other friends either, or that they're even planning regular activities that would make for a normal friendship, which definitely puts a different spin on this whole thing. It is a one-on-one thing everytime. And now they're sneaking around, alcohol being served to minors, and dh is jeopardizing his job by getting doctor's notes. This is not a good sign. This is the deception you KNOW about. This relationship is bringing out bad and dangerous behaviors in your husband that will jeopardize your marriage.
There is also the dad thing to consider. Where is the 20 year-olds dad when all this is going on? You said the kid still lives with dad. Pretty weird that dad is allowing a grown man to monoplize his home and his son's time. Does this kid have a job or go to school, or does he have a silver spoon in his mouth and is living off of dad's money? Or, is there a possibility you and dh are his sugar parents, but you just don't know it yet? Where are this kid's friends and girlfriend? Is he a geek or basement dweller or something? You gotta find out. As someone said, this has "reality show" written all over it. You just don't want it to end like the Housewives of Hollywood or whatever it's called.
Something is definitely up when they're hanging downstairs, and you need to put your foot down. Just like when a man talks about the secretary at the office too much, or stays at the bar too late during the week with friends, most women rightly conclude her husband is cheating. For most, when they see and smell smoke, there is fire.
I think you are right. He IS having an affair, and in your face. I wouldn't be surprised if he eventually confronts you and admits he's truly in love with this guy and wants to do something about it. I think this isn't about your husband not having male friends. I think this is about a man who hasn't been honest with you about things concerning your marriage and family. It sounds like it's only a matter of time before he makes a break. Whatever you do, don't wait for him to come to you about it. Who knows what those two are up to all day. There are diseases to be concerned about and the safety of your children...since you have no way of knowing what they are being exposed to while your are away.
Time to put your foot down and wake up and smell the coffee.