I bought a wheelchair for my son. Can I deduct the conversion portion of the van?

[TomK100]

Yes.  The deduction value for medical expenses varies because the amount changes based on your income. In 2021, the IRS allows all taxpayers to deduct their total qualified unreimbursed medical care expenses that exceed 7.5% of their adjusted gross income if the taxpayer uses IRS Schedule A to itemize their deductions.  ("Car" includes van as well.)

"You can include in medical expenses the cost of special hand controls and other special equipment installed in a car for the use of a person with a disability.

Special design. You can include in medical expenses the difference between the cost of a regular car and a car specially designed to hold a wheelchair.

Cost of operation. The includible costs of using a car for medical reasons are explained under Transportation, later."

For additional information: IRS Pub. 502 Medical and Dental Expenses (page 7)

[Hal_Al]

Yes, if your son is your dependent.

Even if he isn't your dependent, you may still be able to deduct it.  

If your close relative cannot be your dependent because he had more than $4,300 of gross income, but otherwise would have qualified as your dependent (basically that you & other family members provided more than half his support), then you can include in your deductible medical expenses any qualifying medical expenses you paid on his/her behalf just as if he/she was your dependent.