Yeah, he can sue. It'll get tied up in court for a shory while at least, but I don't think that he will have much of a case other than perhaps a pissing match over the dollar amount. Remember that the 5th Amendment requires just compensation, not total compensation, so . . .
If you know a lawyer that would want to take this up, perhaps you should make the match. It does look like they are trying to work it our for the moment with this community meeting, but I don't trust DHS and Nappy to play nice with others in any event.
The just compensation standard has essentially evolved to mean FMV, though the government has way too much leeway in determining what FMV is. That said, that group of beachfront home owners in NC (I think it was NC) were actually able to get the Supreme Court in their favor on a government seizure by showing no legitimate government interest in the specific property. If the farmer could show that CBP doesn't need this much land (shouldn't be too hard, given the number of cars that cross), they would have a fighting chance.
I just took a look at where the crossing is.
You have got to be kidding me with this. They are MAYBE 10 miles from the major crossing at Highgate Springs/Saint-Armand on I-89. There are 2 other probably equally little used crossings between Morses Line and I-89. There is absolutely no legitimate interest in this. Sorry, CBP wanting more comfortable offices is NOT a legitimate interest.