Looks like his full retirement is taxable until he reaches normal retirement age. This is from pub 721.

Disability Annuity
If you retired on disability, the disability annuity you receive
from the CSRS or FERS is taxable as wages until you
reach minimum retirement age, as explained in this section. However, beginning on the day after you reach minimum retirement age, your payments are treated as a retirement annuity and you can begin to recover the cost of
your annuity under the rules discussed earlier.

link to pub 972.  https://www.irs.gov/pub/irs-pdf/p721.pdf