I'll echo Everah's comments that each person has their own preferences. My Father could not be more sports oriented. He was a body builder, played pro (minor league) ball, coached every type of sports team for kids, and so on.
Yet personally, my idea of bliss is a cool room, with low lighting, a large glass of sugar-free tang, and a computer to express myself with.
We are pretty much night and day in every way, I broke the rule and fell FAR from the tree.
But hopefully there is a lesson in my experience beyond the simple "he might not like it". I developed my love of computers slowly, and over time. I took a logo class as my first exposure to computers. I hated moving that blasted turtle, because he kept listening to my mistakes (more than what I meant).
When I made it to Pascal, it was even worse! Now the blasted hunk of metal and plastic would even argue with me over punctuation. PUNCTUATION! What was this, a perverse technology to make me love english class!?
It wasn't until I got my first Commodore 64, and had it all to myself that I discovered basic. Suddenly, the language was clear, and it let me do something amazing, that I couldn't do before: I could make it do what I *wanted*. Not what I said, not what my teacher told me to do, but what *I* wanted.
It was the difference between knowing "Se habla espanol", and knowing how to fluently speak spanish. I could *communicate* with the machine. Instead of it arguing with me about punctuation, in my first five minutes I made it say "I LOVE YOU MOM!" thousands of times on the screen. It was amazing to me that it didn't argue - it just did what I wanted.
Thats what you are hoping to unlock in the young prodigies. It doesn't matter if its music, band, sports, karate, or computing. It all boils down to the same concept - you want the child to discover that their mind, their body, their ability MAKES THINGS HAPPEN.
Once you figure that out, the world becomes an incredible place, full of potential. The only question becomes what you will put your mind to, and how hard you will try.
For me, there was little value in sports. I wasn't the strongest, the fastest, or even likely to become any of those. On a computer however, regardless of anyone else, *I* could make it do EXACTLY what I told it to do. I just had to learn the language.
So focus on making the break-through with your nephew not in terms of specific technology (I mean, html? Big deal, its a poster on the net!), but in terms of getting him to accept the fundamental truth that he can make things happen - using technology.
Lego's are a fantastic starting point, because deep down, thats the lesson in lego's as well - your imagination fuels them. You can make anything if you put your mind to it. Its the same with computers. Start there. You might find that its beneath his abilities, but the message may be more clear.
Teach him that he can do anything, and in 20 years, he may be earning more than you did at his age, working on a technology you don't understand. Thats pretty much what happened with me and my Father.
