First, congrats on a being a stand up single dad. I've been there. My daughter who is now 16 and I have an amazing relationship and we have traveled by Jeep all over.
As for travel, I think its a safe bet Canada is going to be tough to get into for some time. As much as I love Montreal, Toronto, Banff, Lake Louise, Vancouver and even Thunder Bay, I'd have two plans... one Canada and one south. When it comes time to pick him up in August, drive. Enjoy some solitude and see the scenery for yourself. I love traveling solo. So much to see and no one to worry about.
Look at the National Parks Map to see a rough flow of travel. One line goes slightly north along I-90, one goes slightly south along I-40. If it were me, I'd drop down into the Blue Ridge and Shenandoah mountains and then head west to Mammoth Cave. Catch I-40 and bang out some miles and start in Colorado. Rocky Mountain NP is a great place for you to get a real sense of what Colorado's numerous 14,000' peaks are all about. Then either north to Wyoming - Jackson Hole, Yellowstone or west to Utah and Moab, Arches NP, Canyonlands, Capitol Reef, Zion and then to Idaho and Snake River Valley, Craters of the Moon. Either way, then west to Oregon and Crater Lake, Williamette Valley (tons of waterfalls in the area) and then north to Mt Hood (OR), Mt St Helens (WA), Mt Rainier(WA) and not knowing where your ex is, Mt Baker (Canadian US border area). Access to the mountains is terrific. Mt Rainier is the tallest volcano in the US and seeing the devastation left by the 1980's eruption of Mt St Hellns is unreal. Trees as far as the eye can see all facing the same direction on the ground like matchsticks. Its crazy.
All of the places I have mentioned I have been. They are all worth the stops and travel. Selfishly, I'd go north to Wyoming as Yellowstone has so much to offer for a kid your sons age. Utah has amazing topography, but Yellowstone has amazing animals, topography, geology and its not quite as hot as Utah is in June.
Buy a National Parks Pass before you go. Makes travel easier and a lot less expensive.
America the Beautiful National Park Pass - Expires September 30, 2021
Make sure your son gets a sticker from every park you visit. This kind of trip they will add up quickly.
National Park Decals - Shop Americas National Parks
Great maps selection for planning:
Find a National Park Service Map
If you need specifics to Yellowstone, drop me a message. I lived in the park for 2.5 months in 2018. There are definitely some tips and tricks to get the best out of the park.
On a side note... I'd avoid California like the plague right now. Instability on every level is an understatement.
This is a regular occurrence in certain parts of Yellowstone.