For shorts, I love those below from Vineyard Vines. Gap has a nice pair of madras shorts for a good price.
Boys' Madras Club Shorts at Vineyard Vines $69.50 to $75.00.
J.Crew has an excellent selection of madras, of which most is currently on sale. This J.Crew blazer would be really fun for a young boy.
Boys Pacific Patchwork Madras Blazer at J.Crew
$135 now $49.99.
And, this Lacoste madras shirt would look great with shorts or jeans - or dressed up for a special occasion.
Boy's Short Sleeve Madras Plaid Shirt at Lacoste
$80, now $47.99.