I know there is a stereotype that programmers are not social and hence are unlikely to marry, and even less likely to have families. While I do not say the broad brush should be painted on us all, I must say it strikes me that some of the people posting here apparently do not remember what it was like to be a child and possibly have not dealt with young children for quite some time.
As CornedBee and I have been trying to point out, we believe the boy's question is one of reliability and consistency. Can the computer make mistakes? I would say, as a machine, it doesn't make mistakes when working because it's been designed to always be correct in a mathematical sense; If it does make mistakes in math, it's broken. His next question will probably be, "How do you know if it's broken?" Then you can get into aspects of hardware failure and the like, or explain that if the computer was sufficiently broken to render math calculations like simple addition to be incorrect, it is likely that many other things would not work and they would be noticable.
Remember, he's only 5. He's barely had time to use a computer in his short life, and I imagine he's curious how it is his uncle will trust this computer, which appears to have human-like abilities of doing "smart" work of adding, something which he himself as a boy in school is currently going through. He's wondering, if he and his uncle, or whoever else, can make mistakes, and yet they are smart enough to add, what makes the computer anymore reliable than a human?
The answer is that it is a machine.
At first, I was very against Salem's answer because I thought it would attempt to press too much information on him at once. I'm still against it in that regard, but should the computer ever give you the wrong output due to a bug of yours (more likely than the computer actually failing imo), then you could easily explain the nature of GIGO and how the computer, as a very obedient machine, will listen to even incorrect or silly instructions.
To tell the kid that you don't know if the answer is right is silly. He'll take your answer at face value instead of grasping a higher meaning, and conclude the computer either makes random guesses, or cannot be relied upon because it can't think clearly for some reason. At the level we rely on computers, I think their math abilities are quite consistent and something we can -- actually, we now have to -- rely on.