Box turtles normally carry a certain parasite load. Think of all the things this turtle has eaten outside in the last 25 years. Normally this doesn't hurt the turtle unless they become severely stressed (ie. severe illness, or major injury). Your turtle has adapted quite well to living with these parasites.
I think your vet gave you the best advice. I would put it back. This turtle could easily live another 50 years or so in the wild. That is a super long term commitment to an animal if you were to keep it. Plus vet care for reptiles is not cheap. They require very specific living environments that few people, upon finding a boxie, are willing and able to provide. This turtle is used to taking care of himself and has gotten quite good at it over the last 25 years. Keeping a wild boxie in captivity can actually cause them stress and unhappiness. Think of it from the turtles point of view. Would you rather be in the wild as you are used to living or being kept coupted up in captivity?
If you really want a pet box turtle consider adopting one that has been a pet, is used to living in captivity, probably has already been dewormed, and NEEDS a home. Due your research, if you haven't already printed off several care sheets already. If you need help finding information or locating an adoptable pet turtle post that as a follow up question. Make sure you have a housing setup that is appropriate for the turtle to ensure a long healthy, enjoyable relationship with your pet.
Another idea is to buy a hatchling or young turtle from a breeder. It is very exciting to watch them grow and develop.
If you are not ready for 50 year commitment to a turtle (I've had my oldest turtle since 1989.) consider planting things in your yard to attract turtles to come and visit you! It is really fun to watch them forage for food in your own yard. They get to come and go as they please, and you don't have to worry about how to care for them as they can take care of themselves. It is a win-win situation for everyone, especially when their habitat is continually shrinking.
If you'd like ideas for planting things that will attract turtles, you can post that as another question also.