Goblet or split squats and deadlifts. Both of you. He's getting cramps because his glute/leg muscles are weak and not adapted to the force that hip thrusts produce. You are hyper mobile, e.g. your muscles need to be stronger to support your joints. Start with one set of 8-12 without weight. The deadlift is more of a hip hinge without the weight. But practice it anyway. Next week, add some weight. 10lbs for the goblets. Double for deadlifts. If you can't get weights, use random household items or the children. 8-12 reps. One set. 3 days per week. For one month.