The most economical way to keep up with growing boys and their clothes is to use hand-me-downs from your older children or from your brothers, nephews or neighbors. The next most economical way to clothe your child is to swap clothes with someone you know who has a boy a size bigger than yours and/or a daughter a size smaller than yours. You can give your friend your daughter's outgrown outfits and the neighbor can give you her son's outgrown threads. Online swapping is another option, but you frequently don't see the clothes you're swapping; it works that you order a box of, say, winter clothes for boys' size 14, and you pack up a box of whatever clothes you're not using. If the box that you ordered is full of button-down shirts and your son only wears polo shirts, you may have a problem.
Another economical way to buy clothes for the whole family is to purchase from consignment shops or second-hand boutiques. Sometimes you can bring clothes in good condition that your son has outgrown in the hope that the store will sell them for you and will give you a percentage of the money, which you can then use to buy clothes in his size. You can see exactly what you're buying (unlike online swapping), so be sure that the zippers work and that the garment isn't stained. Check out your local Army/Navy surplus store, too.
Still other tips are to buy clothes a size or two too big and hem the sleeves and pants legs or to wait until two weeks before an occasion such as a wedding before you buy your son a suit, since if you get it too far in advance, he very well may grow out of it before he ever wears it. Let your son help you keep track of which stores are having sales on boys clothing and don't be afraid to comparison shop or ask the owner for a discount if you buy in quantity.