If he is not being paid for this, then this would be a charitable mileage donation. If the organization he is volunteering with is listed in this IRS Charity Search Tool as a qualified charitable organization, then you would enter your donation/mileage by taking the following steps:
If you are itemizing your return you will enter these miles by clicking in TurboTax Online
To enter your charitable miles driven click the following in Turbo Tax Desktop:
Itemized expenses include mortgage interest, state and local taxes up to $10,000, medical expenses in excess of 7.5% of your AGI and casualty and losses in excess of 10% of you AGI with the first $100 not counting towards the loss. Your health insurance and all medical expenses are only deductible for the amount that is over 7.5% of your AGI. This means if your AGI is $50,000, then the amount that is over $3,750 is deductible.
Then your total itemized expenses would need to be greater than your standard deduction below in order to benefit from your insurance premium payments.
The 2023 Standard Deductions are as follows:
Blind and MFJ or MFS add $1,500
Single or HOH if blind add $1,850
Standard versus Itemized Deduction
If he is getting paid for this, and it is a hobby, then the mileage is ONLY deductible if you itemize and only up to the amount that he earns. This income is listed under Less Common Income
If he is getting paid for this and wants to include this as a "side gig" then you would enter the 1099-NEC, then walk through and add expenses such as mileage and uniforms. This form would be listed other Other Common Income