The installing of a walk-in tub is a modification of a home that neither Medicare nor Medicaid usually covers. That's because it involves a physical change to the home, rather
than the addition -- through purchase or rental -- of medical equipment or supplies, which Medicare Part B and Medicaid do cover. There a number of types of what is called "durable medical equipment" that Medicare Part B and Medicaid cover that might be useful to you and your son, including a special hospital-style adjustable bed, patient lift, special chairs and other equipment to make use of a shower or tub both easier and safer, special commodes, and many others.
The types of equipment, and how they are rented or purchased with Medicare paying most of the bills, is described in detail in the official online Medicare booklet called Medicare Coverage of Durable Medical Equipment and other Devices.