Since we don't have definite dates, what dates should he put down on the application?
You have to do an educated guess and come up with an itinerary you can support with documentation. Immigration doesn't like uncertainty. Don't ask for less time than you need. I once asked the German consulate in Washington for a Schengen and provided documentation for five days, expecting they would give me the suggested extra 15 days grace period, they did not and gave me the exact period I asked for hence ultimately the visa became useless to me.
Regulation (EC) No 810/2009 of the European Parliament and of the Council.
An extraction from this regulation:
«…the period of validity of the visa shall include an additional ‘period of grace’ of 15 days.
Member States may decide not to grant such a period of grace for reasons of public policy or because of the international relations of any of the Member States
I know that, once they approve you, they give you 90 days, so does it matter what dates I put down on the visa application?
They don't automatically give you 90 days. I have had several Schengen visas which were anywhere from two weeks to one month and with the Schengen visa, unlike the US visa for example, you have to be out of the country by the day the validity expires.