we have tried every thing- taking his gadgets away, sending him to bed, chores
Well, you have tried every punishment and every threat but punishment and threats doesn't seem to be working. So you do need to find something else.
He's not badly behaved at school - nor brilliant but actually school is not the issue. It's the way he us with us.
Then have you considered family counselling with him? The problem isn't just him, if he was the only problem then he would be a problem everywhere. Part of the problem seems to be that he doesn't behave like his brothers and you don't understand why he doesn't behave like them, he probably doesn't understand why either, and you don't know how to cope with that.
It's really not that uncommon to have one child in the family whose personality and needs are different from the other children, who doesn't fit in to the family as easily as the others, and who really needs to be parented in a slightly different way for whatever reason. And when he doesn't get that, you get more rude and unkind behaviour from him and more punishment from you and it becomes a vicous circle . A parenting course is another way to get give you some insight into the way his needs might be different from the others and some alternative strategies to try.
All 3 of them are different but we do expect him to be like them in terms of values and kindness.
Exactly
You expect him to be like his brothers when he relates to you, to his father and to his brothers. You don't know what to do with a boy who doesn't have the same approach to family relationships.
I cannot put up with the nastiness ( you're fat , dads fat, brother is losing hair , brother is fat blah blah blah )
One simple thing you can decide to do is to label it in a calm voice - "that's a put down" and not react further. What it (usually) really means underneath is "I'm upset about something and I want someone else to feel as upset as I do because that's the only way I have to put my feelings across". In the long run he will need to find a better way, but that may mean family counselling, or developing his emotional communcation in some other way.