You don't really need to train him anything to put an english saddle on him and have some fun. If you plan to show, then yes some training comes into play (mostly just to tweak his gaits ad headset a little bit). Other then that I would just go out and ride as usual and of course have fun!
Horses don't really know the difference between western and english, it's just a slight difference in weight. The big difference is in how we ride, sit the gaits, and we ask for a little bit of different carriage from the horse. If you are just riding for fun then do what Zeke suggested and just go out and have fun.
If you want to get into jumping then do some flat work to get the horse ready before jumping. Other then that it's pretty much the same.
Actually, I would be more worried about putting a western saddle on an english horse.
All my western horses took the englishs addle as, "Thank GOD, a little less weight to carry!"
However, I did a full-care lease on a Hanoverian gelding last summer for a jumper circuit, and when I put the western saddle on him ti was like "OMG Mom put a big giant thing on my back!! Wth Do I do??" And then we had a big episode fo him trying to jump in my lap.
After that, I went to Toni's and saddled some of her english horses western (She likes versatility) and they were also not too thrilled. None of them ended up in my lap, but they were a bit disturbed by that giant thing laying across their backs.
Not saying all english horses will react like this, but apparantly all six of those horses do.
The english horses aren't used to a saddle covering that much of their back, while western horses are just used to weight.
Transitioning shouldn't be hard. I wouldn't worry about it.