Hi and welcome to the forums at Car-Seat.org.
The Frontier XT is the Canadian version of the Frontier 85. You are correct that it only has a 65lb maximum weight limit, however, most kids are both mature enough and old enough for a booster by the time they hit 65lbs. The Frontier XT does have the tallest harness height available in Canada, and since most kids outgrow their seats by height before weight, this is an important factor.
There are seats available for special needs kids and those seats have higher weight limits than 65lbs - but they are available only with a prescription from an OT. For regular seats available in the marketplace, 65lbs is the highest weight allowed under CMVSS 213.
To be quite honest, it is a rare child in Canada who reaches 65lbs before being mature enough to sit properly in a booster. Even in the US it's going to be a rare child who actually needs that higher weight limit - it really only exists for the kids who are young and heavy - in most cases a child would have to be quite obese to be over 65lbs before reaching the age of 5. (Though I know some kids are also tall to go with that weight, but that is an immense exception, most kids weighing that much before age 5 are simply obese.)
The only seat which is really comparable to the Frontier XT is the Graco Nautilus. The Evenflo Maestro also has a tall harness height, but it only has a 47lb weight limit and likely would be outgrown by weight.
If you're happy with how your son fits in the Frontier XT and it fits in your vehicle well and you find it easy enough to use, then I'd stick with your original plan. There are some differences in terms of the Canadian Frontier having additional energy absorbing material in the head area in order for the seat to meet Canadian standards, but in terms of the fit of the child in the seat it is going to be the same. :thumbsup:
