Hi Shani! I hope you are doing well
Great to hear you are both doing STS. Here is an alternative exercise that your hubby may be able to do. It is a one legged standing dipping squat movement. Have him stack a 4, 6 or 8 inch high step (or line up the long side of a regular sized step) next to a wall. He can stand on the step so that his right side faces the wall (his right hand will be placed on the wall for balance). and his left side is free to hold a dumbbell in his hand). Have him stand closer to the left side of his step so that he is free to lower his leg off the side of the step without interference.
The Exercise:
One Legged Standing Dip Squat
Standing on the step, have him hold a challenging dumbbell in his left hand with his left arm down at his side. His right foot is planted firmly on the step (so it is completely flat) and his right hand is extended out to the side to hold onto the wall for balance.
Now he should bend his right knee and lower his left foot down as far as he comfortable can and stand back up. As he does this his left foot will lower down and possibly tap the floor (which is fine) but if it touches the floor with no problem, raise the height of the step a bit. The hand weight used along with the gravity of the move, creates a challenge to the quads and glutes. More quads though if he stays shallow in his dip. After 3 sets of 12 he should turn around and work the other side.
How to make it more challenging:
As he gets comfortable and better balanced with this exercise he can let go of the wall and put a weight in both hands as he lowers himself down.
To make the exercise target more glutes than quads, he can do the STS exercise where we sit in a chair and stand back up (holding a weight) with one leg lifted the whole time. If the chair is too low of a range for his foot to allow for, then put a few thick phone books on a chair so that his range of motion is reduced and he doesn't have to lower his body down as far.
Good Luck!