If you're getting quotes of £2500 for a 17-year-old I'm guessing that you don't live in one of the postcodes where insurance premiums are particularly high. There are several parts of the country where a 17-year-old can't get insurance for under £4000.
A few hints (based upon reading hundreds of similar posts on AB!):
1. When your son passes his test he might want to consider going for 'Pass Plus', which can get him a discount on his insurance premium ;
http://www.direct.gov...iedDrivers/DG_4022426
2. It's worth getting quotes for all levels of insurance. It's easy to think that 'third party only' is bound to be cheapest, with 'third party, fire & theft' costing a bit more and 'fully comprehensive being the dearest' but, strangely, it doesn't always work out that way. 'Third party, fire & theft' is frequently cheaper than 'third party only' but 'fully comprehensive' is sometimes the cheapest of them all! ;
3. Many people, here on AB, have reported that they've achieved significant discounts on the premiums for inexperienced drivers by adding another (more experienced) 'named driver' onto the policy. So putting yourself onto your son's policy could see his premium reduced. (That's perfectly legal, even if you've no intention whatsoever of actually driving his car) ;
4. Read this lot:
http://www.moneysavin...surance-young-drivers
Chris