ish, in my experience the very brightest kids don't tend to have the most agreeable social skills.
B students tend to be the ones with a lot more social savvy, and the most predictably even personalities.
The very highest IQ kids don't tend to be like that. They tend to be more edgy, more demanding, less willing to compromise in a social setting. It kind of comes with the package.
Schools who deal with truly gifted and talented children completely understand this. Sometimes schools get derailed and mistake hard working socially adept kids as the gifted and talented, but it's not the same thing. Hard working agreeable doesn't equal G&T.
As time goes on he will learn a little better how to hide his frustration but I don't think you can expect him to completely change personality. Best wishes. He will probably go on to contribute greatness, but may not be the most pleasant guy to be around always. ;D
One thing-- he will probably improve in the meltdown area with age, regardless of anything that you do. He is still young.
If social skills classes were not a success for him, maybe you could try to find a social skills coach that could start off with him one-on-one. Some of the counselors in my area do this-- it might really be a better fit for him if other children sort of "set him off" in that regard.