Congratulations. My Ds is in Y8, having started in the Junior Form and loves it.
There are few compulsory extra costs.
School lunches are available on a cafeteria basis, with payment via a pre-paid card, so boys can buy what they want each day. Prices will therefore vary depending on what your DS wants to eat. I think you can get main meal deals for about £2.50, with sandwiches and other hot snacks for less. If you prefer, you can provide a packed lunch which many of the boys have.
So far, we have not been asked to pay for day trips, unless it is to pay for lunch or something, but again packed lunches are usually an option. There are loads of trips available every year ranging from theatre visits, to skiing trips etc. some of which cost a lot. So far, all have been optional, and my DS has not been on any apart from an annual trip he goes on with the Choristers. We have not yet got to the stage in his education when the 'academic trips' are residential, so don't know for example what happens for a geography field trip. Here is a link to the letter outlining all this years trips for information trinity.parentcast.co.uk/ParentCast/modules/common/php/load_file.php?id=327 As you can see there are often very limited places available, so it stands to reason that most boys will not go on any individual trip.
We have not been asked to pay for any books (text or exercise).
Music lessons: From Sept. 2014 the termly fee for 10 instrumental lessons will be £220 and the termly hire charge for school instruments will be £40.
You may want to factor in travel costs, depending on how your DS will get to school.