Modern hipsters are all about trim fit clothing. They want short tight jackets with low rise skinny leg pants. This contemporary or modern fit is not to be confused with a boys slim fit.
True slim sizing for boys has always existed regardless of fashion trends. A boy’s slim size is for a tall, very thin young man. The waist is typically 2” smaller than a regular and the lengths are longer. A modern, trim fit pant has the same waist measurement as a regular but has a lower rise and a narrower seat and leg.
My question is how cool do we want our boy to be? Boys are not men. They are still growing and most very quickly. Do we want a suit or pant that is on the cutting edge of fashion that he may outgrow after one or two wearings? Maybe we want a fit with a skosh of growing room that he could possibly wear several months from now. Your call.