Has DH skied before - in which case he will know how things work in a ski resort and won't really need a travel agent (by which I assume you mean a Tour Operator) to sort out things like ski rental, lift tickets and ski school. Organising the travel and accommodation is also fairly easy these days.
There are numerous resorts within easy reach of Salzburg airport, and transfers can either be by shuttle taxi (the airport website has a list of shuttle services) or by free public transport.
Easyjet/Jet2/Ryanair/BA fly to Salzburg, then take public transport or a shuttle taxi to the resort. Accommodation can be found on the the resort websites (e.g. Saalbach.com), or the usual accommodation sites (Booking.com, airbnb, VRBO etc) and even the most basic B&B/Pension will be clean and comfortable or look at apartments, apart-hotels or full service hotels.
One issue already mentioned is that many accommodation providers will want to have bookings for a full week, so finding somewhere that will accept a short-stay booking might be more difficult. The UK Tour Operators also mostly organise weekly trips running Saturday to Saturday. Staying just outside of the main resorts may open up more availability - for example, stay in Maishofen or Bruck for Zell am See, or Viehhofen for Saalbach-Hinterglemm. Other places to consider, all within 90 mins of Salzburg include Leogang, Maria Alm, Lofer or Rauris.