Use the file I attached and read it front to back. It will tell you everything you need to know about the IDES system. As to the question on percentages, be really clear, the maximum he can receive via retirement from DOD is 75% of his base pay. Base pay does not include BAH (housing), BAS (food), or any of his other special pays. He can get up to 75% of the average of his high three salary figures. This is based on how much he made over three years. Usually the last three years in service. So if he averaged $3000 for the last three years, the maximum he would receive in DOD retirement would be $2250.
The next thing you need to understand is the VA is providing a rating as well. It will usually be higher than DOD. The thing you need to remember is by law, you can not collect pay from the federal government for the same thing. So if he get's $2250 from DOD and $2300 from VA, it will not add up to $4550. The VA pay is going to offset his DOD pay. The VA would pay $2300 tax free and DOD would pay nothing.
There is an exception to this rule and based on his 9 years of service, he may qualify for CRSC. He would file paperwork once his VA ratings are final to prove each of his conditions were combat related, or a simulation of war. He would fill out a DA 2860 and send his supporting documents (stuff that proves combat) to the CRSC board listed for his branch on the 2860. They will decide what percentage, if any of his DOD pay that can be restored as CRSC pay. This is the only way he could collect both VA and DOD pay.
Please read the file and it will help you ask questions that will help you. It will also take a lot of speculation out of the picture.