First of all, tell your dad he is a great American and we are all support him.
The problem is knowing how much we are talking about spending here. This will determine what sould be recommended or what is best for the money.
The other problem is the "Marines", "Navy Seals" and
"Special Forces" is what is most popular on combat knives. The several I have in mind to recommened has either Marines or Seal in the name or on the blade. I couldn't care less what it is as long as it does the job it is intended to do. But I know the different branches have the pride thing going so these might be out.
Let us know some more info so we can recommend a knife that is affordable and that would fit the need best.