That website does book flights on delta,and usually last minute tickets are still cheaper- I had to fly home from Germany 2 weeks ago last minute because my sister unexpectedly passed away- my ticket for next day travel roundtrip was only 450$.
As he gets higher in the ranks, it will be easier to get leave approved earlier, and a lot also has to do with his job, and his command team. Tell him good luck in his career- my husband is retiring in June after 23 years.