And don’t look past ROM work as very likely being sufficient stimulus right now. I would have this client stand with support (hands on a wall or rail) on one leg and move the other leg through hip ab/adduction, hip flexion/extension, knee flexion/extension, ankle inversion/eversion dorsa/planarflexion. And shoulder ant/post/lateral arm raises to shoulder height or tolerance if there is discomfort on the way up/down. Elbow flexion/extension. Wrist flex/extension both laterally and post/anteriorly. When most movements are doing well and the client isn’t experiencing post workout issues, add light tubing or have the client gently tense muscles during the movements. Progress from there, heavier tubing or more muscle tensing. Eventually you may be ok to add a load like dumbbells or a cable machine. I would also really encourage the water exercise. At that age, water exercise can be phenomenal.
Have you assessed his overall range of motion? Are you having him work on his ROM at all of the joints? Have you consider aquatic exercise? I would check with his physicians to see if myofascial release is an option. All things should be done with slow progression. Without meeting him and getting feedback from his healthcare team, that is all the suggestions I would have for you.
I would start with SMR on the plantar fascia, calves, and hamstrings and then do static stretches for each muscle group.
I would have him sit in a chair and roll each foot over a tennis ball to try and relax the muscles in his feet. Still in a chair positioned close to a wall, I would stretch the muscles in his feet by extending the toes against the wall while the rest of his foot is on the floor. This places his toes at near a 90 degree angle to his foot. Next have him sit on a bench with one foot firmly on the floor and the other leg resting out in front of him on the bench. Use a tennis ball or even a half foam roller to SMR his calves and hamstrings. Then gently stretch the calves and hamstrings.
I would strengthen his glutes by having him sit on his hands and then push his heels into the floor and squeeze his glutes as if he were about to stand up. Don’t have him actually elevate from the chair but just do reps of pushing through the heels and squeezing the glutes. His fingers will provide feedback to him as to whether his glutes are firing or not. This is a good way for clients to learn to fire their glutes.
To work his quads have him sit down and stand up from a chair positioned in front of a Smith machine with the bar placed so he can pull on it for assistance in standing if he needs it.
A NuStep would be a good way to work on his cardio while also strengthening his quads, glutes, and upper body as he pushes and pulls the handles. If you do not have a NuStep then I woul have him use a recumbent bike.