Sorry to hear the news, but let me see if I can help. I will be blunt and quick and for that I apologize.
You are talking about fitting a stump after he has built up some muscle in his thigh. This sounds to me like you are talking about muscles that either: 1) used to cross the knee or 2) attach in the mid to lower thigh where the prostesis will be placed. If that is correct, then the following applies. If not, say you need to build up control of the hip joint, so he has better control of his stump, then the follow would not apply as much.
There will be the following muscles in his stump that used to cross the knee joint or attach in the mid to lower thigh: quadriceps, hamstrings, adductors, tensor fascia latae & glutes (both via the ilio-tibial band). Let's look at each of these a little closer.
* Quads - of the 4 heads of the quads, 3 will atrophy to negligible muscles no matter what you do - the vastus intermedius, vastus lateralis, vastus medialis. The rectus femoris will survive and can be built up.
* Adductors - of the 5 parts of the adductors, only one part used to cross the knee (gracilis) and 2 attached in the mid/lower thigh (adductor magnus & adductor longus. The exercises will be similar for all 3 of these.
* Hamstrings - a part of this group will atrophy no matter what you do: biceps femoris/short head. The other 3 parts of this muscle group will survive and can be exercised: biceps femoris long head, semi-tendinosis, semi-membraneosus
* Sartorius - this muscle will survive.
* Tensor Fascia Latae (TFL) and glut max via the ilio-tibial band. Both of these can survive and be exercised, but they will not impact the circumference of the stump significantly. The muscle bellies are in the hip region where the prostesis will not likely attach. Exercising them will have some impact on the shape of the stump however, so these should receive attention as well.
For exercises:
The easy solution is if you can attach some type of strap to his stump and connect it to a cable weight lifting machine. If that is successful, then exercise the stump in 5 different directions.
1) Straight forward - with his back to the cable
2) Straight backward - with his face to the cable
3) Straight sideways (laterally) - with his stump away from the cable
4) Straight sideways (medial) - with his stump towards the calbe
5) Diagonally - Starting with his stump down and medially, raise it up and to the side while facing the cable.
If all 5 of these can be done standing, great. If not, try each in a lying down position.
If you cannot get a strap to stay on his stump with these exercises, things get a little more complicated, but not really difficult. All of the above exercises would need to be done by stabilizing his stump to a weight bench and then moving his body so that he achieves the same 5 motions. With each of these exercises, it will be critically important that his pelvis moves. All of the muscles that we are talking about exercising attach to the pelvis. If he only moves his torso from the waist up, without pelvis movement, the muscles we are talking about will not get exercised.
For example, to exercise the rectus femoris - Lay supine on weight bench, strap the stump down to the bench and let the other leg rest on the floor without contribution to the exercise. Do a sit up, making sure to come up all the way, with lots of pelvis movement at the top of the sit up. The rectus femoris attaches to the front/top of the pelvis so unless the pelvis moves, the rectus will not get a workout. (i.e. sitting up partially, rolling up the abdomen without pelvis movement, will only use the abdomen muscles but not the rectus femoris).
Hopefully, you get the idea from here. The other 4 moves would be done in a similar manner. 2 varieties of side sit ups with lots of pelvis movement and a back extension (with lots of pelvis movement) and a diagonal sit up (with lots of pelvis movement). It may be easier to do some of these exercises with a variant of a back extension bench instead of a straight weight bench. The type with a small pad to rest your stomach on and a foot plate and a roller to place your other feet on. Where the amputated foot would go, it would be essential to weld a new piece in place that his stump could be strapped to.
Good luck.
