(Been gone a while)well, tb, being able to grip with your thigh, and gripping constantly are 2 different boats, in my book. Maybe it's just that my horses are cross trainers (lol) and when we're going western or dressage (english work is limited, so I guess I am used to longer stirrups), unless you're giving an explicit cue, I was always taught to just sit up there like a fly on their back, not gripping - it may be because I learned bareback first, as a kid. When I've been training, I can't get away with squeezing (even with thighs) for a spook. It always seems to push them further over the edge. The western horses I've ridden & worked with (unless they don't know there leg cues - boy are there a lot of those suckers) go faster when you squeeze anywhere on the leg. The seat is so deep, and balanced that if you're in there securely, you don't need to grip. Part of that for sure is the longer leg. BUT, we may just be having a 'discipline' miscommunication here. Video speaks wonders. Lol. When I'm galloping, I only give pressure with the thigh to knee when I'm asking for more speed. My jumps have all been over cavaletti and logs, and I just stood up & leaned forward a bit for those (probably not right, but western saddle and all, at the time *cough* well....) When I get a rear, I will grip, just instinct I think.... moving along. Now, The simplest way to ask for a lead change is just to step down in your stirrup. They will move under your weight, and viola! Lead change. Lol. For flying lead changes, do MASSIVE figure 8's, and as you come up to the crossing point, trot them for a few strides, then cue to canter and ask them to tip the opposite hip towards the lead ( if you want a left lead, cue with your right leg to move that big butt to the left) . Chances are, that western feller you're working with hasn't had his buttons pushed in a looong time. They're there, they're just ... rusty.Now, someone like him, you probably grip/squeeze the crap out of all day long and not get much of a reaction. There are a TON of western horses like that. Seems like a lot of 'weekend' riders are lazy as hell, and love those horses that don't take leg cues. They also love when they just stick their nose up the next one's tail. However, they are all started the same as youngsters, with leg cues. It's not until they really get it do they switch over to neck reining. Try some ground cues with him to see what he remembers. It may take a little coaxing to get him to do it again.
Ask Often, Demand Nothing, and Reward Generously.