What my instructor always taught me was to visualize all of your weight just sinking into your heals. If he's sitting correctly, his weight will bring his seat deep into the saddle and deep into his heals. I then visualize keeping my legs soft and wrapping them around the barrel of the horse. Um, those wooden dalls that have joints (Amazon
.com: 4-1/2 Inch Wood Artists Manikin:..."> Amazon.com: 4-1/2 Inch Wood Artists Manikin:...
these). I use these as a visualization too. I visualize a nice hinge at the knee, that allows the leg to move freely. If I'm gripping with the knee/upper thigh then the lower leg will be swinging up behind and out of the stirrup. It's a natural thing for a person to do. However, if I keep that knee soft and able to swing the upper thigh and lower leg can work together and allow my weight to sink into my heals for anything out of the saddle (such as trotting/cantering/jumping). It allows my leg to extend and my heals to sink further down.
That may sound jumbled and confusing, but that's how my mind works and it's served me well at least. GL with helping him!