The dreaded eBay is probably one of the best places to sell your cards but the fees are creeping up and it is quite labour intensive. Make sure you pay the extra 5p to make them visible on US and Canada eBay sites. However, the market on eBay is very much swayed towards rarer cards and I do not know if these are in your son’s collection. Not many Pokemon cards appear in physical auction houses and rarely make much money. I notice Toovey’s auctioneers have specialist Paper Collectable auctions that include collectors’ cards. After that, you are probably looking for a local game shop that organises Pokemon tournaments, but again, they will only really be looking for rarities. Going for this latter option though you will need to have an idea of what you are hoping to achieve, whereas auction (online or physical) at least the market will help decide what they are worth.
Hope that helps!