First of all amazing job to Milos tonight. I have no idea how he played with so much composure in his first ATP tour final.
Regarding changing nationalities, I really don't think he would. He's lived in Canada since he was 3, went to school here, trained here, and has family/friends here. As Canada's clear top player he's going to have way more opportunities than he ever would in Montenegro. He's also talked about how he wants his results to help with tennis development in Canada. For me it would be a shock if he were to ever play for Montenegro. Plus, being a Canadian is awesome anyways. Why would he want to?