If he is happy, I would leave him in his current grade. In our district, there are always kids who take Algebra II while in 8th grade. They just get on the high school bus in the morning and take the math class 1st period, then the bus takes them back to the middle school. If that is possible in your district, it is a reasonable solution.
Maybe this isn't the solution, but if he is on the computer all the time, maybe he needs more activities? You said he wanted to play baseball in HS. Some areas have strong rec baseball programs, but around here the rec baseball/softball are weak. My older two have played travel ball for several years. Does he play travel ball? That would get him away from the computer (though my kids still find ways to mess around on the computer for hours, despite travel ball, club volleyball, Model UN, tutoring at the local church, etc.) If he plays travel ball, you will spend lots of time with him (some in the car driving to practice and tournaments, and time to explore some different towns between games at tournaments).
I would also tend to wait if he wants to play HS ball. Even though I have girls, the difference I see in skills and strength from age 12 to age 14 is huge. A good, positive experience in sports is important to my kids and they would have been at a disadvantage if they were only 12 and playing HS ball.
