It would be difficult for your brother to get an insurance policy of his own with his foreign UK license since most insurance companies require a US license. If he plans on staying in the US then he can apply for a US license and probably an insurance company will start an insurance policy with him as long as he is able to provide a US license to them within a short period of time around 30 days normally.

If instead your brother is in visiting you, then you may want to see if your insurance provider will allow you to add him to your current insurance policy for the time he is in the United States and Canada driving your vehicle. Also make sure that your insurance company will extend your coverage to Canada. Normally US insurers will. If this is the case and your brother is put on your policy he should obtain a yellow Non-Resident Inter-Province Motor Vehicle Liability Insurance Card from your insurance company.

This card indicates that your insurance company has agreed to provide coverage of not less than the minimum legal requirement in any jurisdiction in Canada in which you travel. Keep the card in your vehicle as proof of your insurance if requested by a police officer.