I purchased my 2017 Volt LT in May. The only time the gas engine wasn't up to the task was while driving east (downhill) across Kansas at 78 MPH with a 30+ MPH head wind. I put the car in mountain mode when I got on I-70 just west of Limon and by the time I made it to Kansas City I had depleted the battery. This was 400+ miles of head wind for an effective 108+ MPH wind speed drive which is actually faster than the car's 102 MPH road speed governor. Coming back across Nebraska (west & uphill) I had no wind or issues at the same speed. I don't think you'll have any issues - Kansas is somewhat unique in that I seem to always drive into a headwind, east or west. Also, the car has more than sufficient power in Normal EV mode to charge up I-70 west of Denver, which is a 10 mile long 6 to 8% grade starting at 6,000 ft. above sea level.
Yes it's not as big as some other cars. My previous car was 15.25 cubic feet of trunk space so it was an adjustment for me as well. Drop the rear seats and you can get a lot of stuff in there however. I haven't taken it to Costco yet but only because I haven't had the need. I wouldn't think twice about doing so, however. As with all hatch and lift backs, the one thing you have to watch out for is not loading the cargo area too deeply. I have met people who broke their hatch/lift back window by too deeply loading cargo. The Volt can easily handle 24 inch deep cargo all the way back to the end of the cargo area. I was pleasantly surprised at how deep the cargo area is.
Go sit in one. My visual estimate is that it's bigger inside than the Honda Fit. The front seats can go a long ways back if needed. What's really nice about the Volt is the steering wheel is not only tilt but also telescopic so you can do what I do and push the seat a long ways back for your legs and then pull the steering wheel out to be comfortable for your arms.
I've had two adults in the back seat of my Volt with no issues. Also since the Volt is a 5 passenger car it has a seatbelt and latch points for car seats in the middle of the back seat area.
Your best bet will be to go test drive a Volt. Take your son and have him climb in the back just as you would normally go somewhere with him.