This is just my opinion, so if you don't like my ideas, you can just ignore me and I won't be offended. :) (As a precursor to my answer, I homeschool my children through K12 and my 7 year old is a year ahead in all of her classes, so again, academically gifted.)
I do not put a "limit" per se on my children's screen time, be it TV, Nintendo (Wii or DS), computer, etc. We just use common sense, and every day is different. There may be some days where everyone in the house has a cold and all we want to do is lay around and watch TV after school...so that's what we do. Whenever she gets a new game, particularly if it's a Mario or Zelda, she will want to spend more time than normal on gaming...which is okay with me. I am an avid gamer.
An IPad is an awesome gift, but I want to encourage you to at least consider some sort of real gaming system if you're going to allow him to do these types of activities. Games don't always have to be "educational," and you'd be surprised at how truly educational the NONeducational games can be. Basically, I don't my want my children's "fun" time and "game" time to just be an extension of their education, because I feel like I'm giving them an excellent education. I want them to have fun.
Again, I still think it's an awesome gift, and you know your child best. I'd tailor it solely on who he is, what he does in his free time, etc. Mom knows best!