Suits

Most suits can be ordered, received, and altered in the span of a month. I would shoot for the 3 month range though! With your 11 year old, if he's starting to hit a growth spurt, maybe wait until closer to the date to get his pants hemmed or something!

I agree with Samantha. Also, if you're going into a store to purchase the suit and do the fitting, often times they will be able to tell you a recommended timeline. You probably want to wait as close as possible to the wedding since at that age they can be growing quickly, but you want to doublecheck that the store/seamstress responsible for ordering/tailoring the suits is able to accommodate the timeline you plan for in order to avoid surprises.

When we ordered our groomsmen attire, I believe they said not to order anything for kids more than ~2 months out. I'd echo talking with the place you're planning to order from to see what they recommend based on their ship times, tailor availability, etc.