Some people are introverted and homebodies. Not everyone is like you nor your children. I wouldn't see a parent allowing their child to remain living there for as long as wanted a problem at any age..especially if they're no trouble..both get along, and enjoy each other's company..
Not everyone is capable, financially stable, can handle living away from home, nor prefer to move out.
Maybe he's also perhaps depressed and having a hard time in life.. perhaps this is also how he copes with life.
Personally, and in my opinion, I wouldn't find this a big deal. You know his routine, come up with a different solution that doesn't involve being cross at both, trying to get rid of the son, trying to get your husband to toss out his child..
You have other rooms that you can go in.
You sound controlling. There are a bunch of other options and solutions, but you're pouting because you want things your way or no way.
Go in another room.
If you want to spend time with your husband alone, both of you need to make time...and make time somewhere else indoors or outdoors.
I just don't think that you like his son, and you just want him gone.
You're shaming him for living in his house, relaxing, eating, drinking soda, his body performing natural things, and watching telly.
You haven't a problem with your own children living in the home, and shuffling about.
Or if you really can't deal, just end things and leave.