Did you notice that you didn't say one single positive thing about your son? Not one. You used positive, glowing descriptions of your other children but not your son. You sound as if you dislike him intensely. He's a first grader, so he's only, what... 6 years old? Almost 6 years old? Yet you described him as selfish (no kidding, really? he's a little kid), wild, vulgar (what?), and hyper. Clearly you think he's "bad" because of his behaviors on "yellow" at school and hurting his siblings, as well as the toileting issues.
You're looking for there to be something wrong with him and have taken him to specialists who say there's nothing wrong with him. That speaks volumes. It means it's not the child that needs fixing. It's the parenting.
He has to share time with how many siblings? and now he's going to have to divide that time further with yet another siblings. The only attention he gets is negative attention, but it's still attention. Therefore, he's going to keep on keeping on with what works for him.
If it's not working for you, then YOU have to be the one to change. Change how you spend time with him. Change how much time you spend with him. Change how you discipline him. Change the words you use to describe him and try to make them less negative. Don't let him hear you describe him negatively. Don't let him hear you express that you believe there's something wrong with him. Positive language and positive parenting can make all of the difference in the world.
And remember that just because he's the eldest of your children that doesn't mean that he should automatically "know better." He may be bigger than the other children and more verbally capable, but he's still a small child who needs his mommy. He needs her love and attention and understanding. I would strongly suggest family therapy, parenting classes, and perhaps getting together with the school and requesting a Behavior Plan. Advocate for your son so that things are more positive for him. He sounds miserable.
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My toddler and preschoolers are calm and well-behaved, usually. The girl of the bunch shows an impressive amount of empathy. My first-grader, however, is selfish, wild, vulgar and hyper. He is usually on yellow at school for talking during class. His academics are so-so as he doesn't ever want to do any. He makes no effort to use the potty at night and soaks through even his pillow (how, I have no idea).
At home each evening he literally bounces off the walls and crashes around the couch, panting. (A full evaluation by various specialist at the hospital showed no issues). These days the kids are always getting hurt or fighting, and it's the 6-year-old! When he's at school, they play well.
If it's just him alone at home, he's fine (though often bored)! But reality is he lives with his siblings.
Last year he did summer camp all summer (switched it up so he did biking, fishing, swimming, nature hikes, sports, etc.) He said he didn't really like any of it and would rather sit home.
So, summer camp again this summer? We have a new baby on the way so I won't be getting out much. I figured a summer camp with friends would be more wholesome! During the school year, it's 3 to 8pm that are tough, but summer it would be all day, every day. It's like he loses it each evening, and eating or not makes no difference, but he can't go to bed at 6pm. I also don't want to stick him by the TV all evening, which would make him super calm (I think he's the type to get addicted to electronics).
Note he fits in fine at summer camp (isn't expected to sit like in school, obviously) and I feel guilty when he's at home all day bouncing off the wall getting in trouble. He enjoys being with friends, too (despite him saying he'd rather sit home). His siblings, on the other hand, can play for hours, color, do crafts, make forts, etc.
Added: Adding to the family is a wonderful thing so I don't mean to sound like I wouldn't add another child simply because one tends to be high energy. Each year gets that much easier, with more joy!