Join Date: Apr 2011
Location: South West England
This means you don't have enough outside leg and rein, allowing him to drift through the shoulder!
When you ask him to turn, you should ask with your seat, keeping both legs on, gently guiding him round the corner. Your hands should stay level, ask for bend with gentle squeezes down the inside rein, but balance him and stop the drifting by still keeping an even contact on the outside rein!
Also lunge him with a pessoa or bungie to build up his topline and increase suppleness, this should make him less inclined to evade by drifting through the shoulder.