Have to buy a suit for DS(17) and haven't a clue what makes a good fit... HELP

[mathanxiety]

Just that DS (almost 18) needs a formal suit for an occasion that's coming up. We went out yesterday and he tried on a few that were an ok fit on the shoulders and the sleeve length seemed fine on some of them. But as for length of the back what constitutes a decent length for a jacket? And where exactly should the sleeves reach to for a good fit there? He fits into a mediumish men's size depending on the brand.

Whatever trousers he gets will have to be altered no matter what so I'm not worried about that end of things. But I'd prefer not to have to pay to have a jacket altered as well.

Had to buy a suit for a 16yo last year. M&S had fab suits. They do a slightly slimmer fittted one that looked very nice and 16yo DC approved and really liked it.

Jacket should be about crotch length/cover the bum. Any vents should lie flat and not gape.

Sleeve length - a half inch of shirt cuff should show. I think the bottom of the jacket sleeve should just break on the base of the thumb (not the V of the thumb and hand but the actual base of the thumb).

Hope you find something you both like

Thank you, thank you -- I feel more confident now. We'll head out for another shot at it today.