Most Helpful Guy
If its his first suit get charcoal, for one thing.
You can't buy a suit without knowing a massive number of measurements. Even when you buy a suit in your size, you still get it adjusted. There is no way you will be able to buy a suit and had it ready for him to wear. Some parts on a suit aren't even finished when you buy it - they are left till you purchase it.
Could you buy a suit, keep the receipt and then have him go, get the right size and have the alterations done? Probably.
For reference, sizing is theoretically off chest size, though the size I wear doesn't match my chest size. It ends up being driven heavily off your shoulders. If you're quite tall or short you need a 'long' or 'short'. So I wear a 42L. But I need waist slimmed sides slimmed sleeves adjusted pants hemmed - assuming its a suit that is a good fit.