Walker (Texas DMV) v. Sons of Confederate Veterans holds that
Texas’s specialty license plate designs constitute government
speech, and thus Texas was entitled to refuse to issue
plates featuring SCV’s proposed design...
When government speaks, it is not barred by the Free Speech
Clause from determining the content of what it says
The plate "NULL" falls into the category covered by this ruling. The court has
'refused “[t]o hold that the Government unconstitutionally
discriminates on the basis of viewpoint when it chooses to
fund a program dedicated to advance certain permissible goals, because
the program in advancing those goals necessarily discourages
In the aforementioned case, the viewpoint that was not permitted was arguably a pro-Confederate viewpoint, and it was ruled that the government has no obligation to express such a viewpoint.
The court found that strict scrutiny under the Free Speech Clause is not applicable in this case; in a potential application of strict scrutiny to the instant circumstances, the government is actually on even stronger footing, since there is a compelling government interest at stake (the ability to bill people for road usage without the need for toll booths). Governments have long been able to restrict insulting and profane words as vanity plates. I would be very surprised if he is able to force the government to accept this plate.