Professional urban planner here...
I stopped reading the reply posts as some people were hinting at some considerations, but I'll restate them quickly. First, negotiate for land for a flag lot for yourself. That way the DOT is unlikely to deny him access to the 3 acres from the road. Your lot has a right to access and so will his. Now he might like to share your dirt driveway, so who knows. Second, IT IS BAD PLANNING TO SHARE A RESIDENTIAL DRIVEWAY FOR COMMERCIAL PURPOSES. Your local planners should know this. That easement was granted to you for your purpose, not yours AND theirs. Thirdly, this does sound like spot zoning. What's around you? Is it ag and homes or commercial businesses? If its businesses, then that's a life lesson not to buy a landlocked lot surrounded by commercial, however that doesn't sound to be the case. AG zoning is transitional until land use trends are established. The fact that you built a house surrounded by agricultural land establishes residential land uses in that area more than his desire to surround you with commercial. Unless the future land use plan says otherwise, then again, life lesson. He bought it before knowing that he could have his business, that may very well end up being his life lesson.
If you're not opposed to the commercial zoning beside your investment, then use some leverage. You have a very good case or him to have the whole thing denied, BUT your indifference to it and especially support (in the eyes of the decision makers) should give him the green light. So use your leverage. Get your flag lot (at a discount even), and tell him that you won't protest his request as part of the deal. The board should not grant the approval if they are aware that the DOT is going to make a commercial property share a driveway with a SFH.
I'm here to help.
Kyle