You could always remind him that if he throws a tantrum, he won't have privilidges for he whole next day. Another thought is rewarding him 15 minutes of computer time for every 1/2 hour of playing outside, or reading a book, or something you want him to help you with.
Good luck and God Bless.
I think he is growing up a little bit. Puzzles and legos may still be fun, be he has discovered something else exciting! What does he do on the computer? Is he chatting, or playing games? Remember, you are the one in charge here! Keep in mind that while you may not find the computer that exciting, he does. You can use the computer time as a tool for parenting. Remind him that the computer is a privilege that has to be earned. Also remind him that it's not a toy and that if he cannot act mature enough he will lose said privilege.
=)