First, and most important, question: What does HE want to wear?
This is a great resource, with a section on suit basics:
http://www.blacktieguide.com <– read this before going anywhere near a Tuxedo.
What time of day is the ceremony? With those dresses he should be in a suit, at a minimum. If it’s an evening ceremony, Black Tie would be better, and even White Tie would not be out of the question, if you’re having a seated dinner. If it’s a day time ceremony, I vote for a Morning Coat, but a formal suit would do well. For a suit, that means solid navy, or solid charcoal, ideally with peak lapels, but notches are ok, and more versatile for wearing after the wedding. Since it looks like no one is actually wearing purple, either colour will be fine.
If you’re going to buy, and it will be his ONLY suit, then it should be charcoal with notch lapels. In terms of what will be best to own after the wedding, If it’s going to be his first of two suits, get navy with a notch lapel, and add charcoal for his next suit. If it’s going to be his first of a few, try to get navy with peak lapels – there should be some around, as they are quite fashionable right now. If you don’t like navy for the wedding, nothing wrong with buying charcoal first and navy second. I bought a steel blue suit before a navy suit because it happened to look amazing, and I don’t need it for job interviews in a conservative city, so I could get away without a navy suit temporarily. This paragraph is mostly advice pertaining to what suits are most practical to own, and therefore the best investment.
Some notes about vests: they should completely cover the waistband of the pants, and should not be worn with a belt. If you can see shirt and/or tie between the vest and the pants, you’re better off in a two piece suit with no vest. Also, the bottom button of the vest, like the bottom button of the suit, is never fastened. This is different for Black Tie and White Tie. For morning dress, it is never fastened on a single breasted waistcoat, and on double breasted is a matter of preference.