I would say most of the UM's we had were traveling between parents, who were no longer together.

In training, we were read the riot act about UM's. One issue was that the child was entirely our responsibility in transit. I used to "assign" a nearby Flight Attendant to each UM (or sibling group). Most of them were little vet flyers and really didn't need much physical help.

On one of my rare domestic, the airline even let the grandmother come on board and sit with her crying granddaughter to say goodbye (that won't happen on int'l but just to show how accommodating they can be).

We were strict with them though. Some were not happy about the trip and would try to slip away. One told me that she "needed" her passport. Please warn him about this. The crew has to keep it.

Once, a UM (not on my flight) did manage to get away. He was slipped off the aircraft and got as far as immigration, passport-free and they were on the watch for him. We all got notes in our boxes about how this was not to happen again and how important it is to keep watch over our UM charges... This child did this on purpose and was never in any real danger. They knew he was in the terminal but even then, they took it seriously.

I have friends whose children fly as UM's quite often to see the other parent. They recommend that you check for the flight status yourself. They were notified by the airline of delays but not as quickly as it appeared in the computer.

Unless someone suggests differently, what I'd suggest is that you put a huge layover time in New York. Meet him outside Customs as if he were arriving and then check in together for the Montreal flight. But talk to the airline(s) and see if they can set it up another way. Listen to them first but in the absence of some sort of procedure for meeting a UM en route, give yourselves enough time to cover for some delays.