Use pie charts to calculate equivalent fractions; it's simple and your child will understand the concept more easily if you make it more approachable for him. Using 1/3 as an example, draw a circle and divide the circle into three sections. Color in just one of the sections. Have your third-grader pretend the circle is a pie and the colored-in slice of pie is the piece he gets to eat.
Explain to your third-grader that the pie is now considered a fraction because it is cut into three pieces and one piece is being eaten.
Draw a second circle, divide the circle into six pieces and color in two of the slices. Explain to your third-grader that the second pie is the same size as the first pie even though it is cut into more pieces. The second pie shows 2/6, which is equivalent to 1/3.
Using your drawings, explain that the colored-in pieces on the two pies are equivalent fractions because even though they use different numbers, they are still the same size. The second pie is the same size as the first, it's simply cut into smaller pieces, which means that it takes two pieces to equal the same size as one piece of the first pie.