Does your employer have an Acceptable Use Policy (AUP) which covers employee use of computers? It might contain terms like Prohibited activities include: Circumvention of technical, administrative, or process controls" which he is breaking.
Depending on what your company does, how big it is, how casual everyone is, all matters. But if you as a company work on sensitive information, then your contracts might require that all workstations have up to date antivirus and patches, and if you can't connect remotely then you can't audit and uphold that.
If you have software that's very fussy about versions, if you can't connect remotely then you can't make sure every computer has the right version and it puts the company at risk of data corruption.
If you have any kind of specialist, expensive software - medical, legal, CAD - it probably has quite strict licensing agreements, and if you can't connect then you can't audit to make sure all software is licensed and no unlicensed software is being used.
If he is abusing the rights granted to him, and in any way putting the company at risk by doing so, you probably should raise it with your manager, his manager, or HR with a view to reminding him of the AUP and why it applies, regardless of whether you can find a clever way to force a reboot.
And you might find yourself responsible for enforcing the AUP, e.g. blocking his access to file shares and email, locking his account.
If none of the above applies and it's a very casual workplace, block his account and access to email. When he can't do any work and his manager starts to notice, and he calls up for support, ask if he's tried rebooting, and make it work again when he does.