Just stick to working with his tissues to bring some relief.
Don't openly question his doctor's recommendations regarding the arch support, as you don't know what information his MD is working with. Hard arch supports may well be a better choice depending on specific pathology and seforum.xxxe of recovery. If you suspect that his arch supports are inappropriate, refer him back to his medical pros or suggest getting a second opinion from another qualified medical pro.
I am actually quite surprised he hasn't been to physical therapy, and I'd recommend that he explore the possibility. A good PT could work with him in ways that you don't, and probably speed up his recovery.
For bodywork, I strongly agree with working the whole foot, ankles, knees, legs, hips, etc. He'll have lots of compensatory patterns by now. Go slow and gently in the areas where he has had the most damage. Get as much info on extent and location of the injury as possible.
If you know how, take some time to work with his interossei, both dorsal and plantar, in both feet. Many feet suffering from chronic pain tend to have loads of issues with the interossei.
Check for signs of hypertonic calves too. You may need to do some stretching of gastrocs and soleus. If you can, teach him how to stretch them properly between sessions. Stick to AIS, PNF, or CRAC stretching methods, these are reasonably simple and safe. The old static stretching approach is probably not going to work well for this client, and it would be easy for him to overstretch and cause more harm.
Last, see if you can get this client to provide you with the doctor's contact info, and get permission to make a call. If you can, introduce yourself and let him/her know you're starting to work with this client. Learn all you can about the specifics of the injury and what the physician thinks is going on. This simple step can save you loads of guesswork and may serve to establish a measure of professional courtesy that could turn into referrals later.
Have fun!