He should be pretty good about switching over. I think that it would be easy for a western horse to switch to english than an english horse switching to western.
When you ride english, you use direct rein pressure. Which is when there is always contact with the bit. Your stirrups are a bit shorter. The cues are pretty much the same between western and english riding.
Lol, yes. When switching to an english saddle from a western one, it can be a shock! lol, I was riding western before I took english lessons, and when I sat in that english saddle, lol, it felt like I was sitting on a little slab of leather!! lol! You will probably feel very insecure but you'll will adapt. You also use different leg muscles. After the first week of english riding lessons, my legs were soooo sore!! lol
Opportunity is missed by most people because it is dressed in overalls and looks like work. ~Thomas Edison