The first thing you have to do is make a shortlist of all the places you can take him to that do not require a visa or are the easiest to get one for. This will narrow your list down considerably.

Then remember to try and think like a Thai when you chose a place as my experience has been that what we would find interesting or fun is as boring as hell for some of them. Also remember he needs to come back and raise his status with his friends as having been somewhere really cool and not just down the road.

As far as UK and I am sure the USA, as numerous posts will tell you, the visa is all about money and security in Thailand that he will have a desire to come back to and not want to stay in the US and UK.