If he is eligible for a regular retirement under FERS (vs an early one), he probably won't qualify for a disability retirement.
In FERS, a regular retirement would be Minimum Retirement Age + 30 years of service, given his age of 56.
If he is eligible for retirement under a VERA, he still might be able to file for a disability retirement. He does not have 30 years of service, per your post, so his potential annuity will be less with him not being able to get in 30 years or work until age 60, when he would have qualified for a regular retirement.
However, in an early retirement under FERS, he becomes eligible for the special retirement supplement at his MRA. Check on whether he will receive the special retirement supplement if he retires now versus having to wait until age 60 for the supplement.
In his case, it might pay to file for the VERA and then try for Social Security disability benefits. If he goes for a FERS disability retirement, even though the monthly amount initially may be higher, there are offsets for Social Security disability benefits. There should not be any offsets when he merely gets a regular or early retirement under FERS.