The best way to explain square root is to start with square numbers. If your brother already knows about the magic of square numbers, then jump ahead to step 2.
1) Explain what a square number is, and how to square a number. You could say something like "To square a number you multiply it by itself. For example 2 squared is 2x2. 3 squared is 3x3. 7 squared is 7x7".
2) Square root is a tad trickier. Start with a perfect square (Some are : 1, 4, 9, 16, 25, 36, 49, 64, 81, and 100). Explain that the square root of a number is the number that you multiply by itself to get the perfect square (that's a bit wordy and confusing though). For example,the square root of 4 is 2, because 2x2 is 4. The square root of 9 is 3 because 3x3 is 9. When you square a number 'x' and get 'y' then 'x' is the square root of 'y'.
Keep using examples, and try to reword what you say. Examples give your brain something to remember, and latch on to. If your brother doesn't understand what you're saying, changing your wording could help. It may just be that hearing what you say differently is what he needs.