Good luck! Cantering is super super fun.
I'm absolutely terrified to canter because of some really bad experiences cantering when I was just starting out, but somehow I ended up with a horse that would never ever think of doing anything silly while cantering.
Before I knew this about her, I'd get all nervous and worried and since she's super in tune with me, she'd refuse to canter. Eventually I just decided to do it and I started forcing myself to get at least 3 canter strides per ride.
Over time I realized that her canter is really pretty comfortable and that I could really just drop the reins and let her go if I wanted to. She only gets strong when she's really looking forward to cantering but before she really takes off, she bounces around a few times and if I yell at her angrily, she'll immediately stop and go back to a nice canter. And, if she's cantering along nicely and I say "easy", she'll immediately start trotting and if I say "ho", she'll slam on the brakes so hard there would be sparks if she had shoes.
Funny thing is that she is so hot in everyday life that before I really knew her, I thought for sure that asking for a canter was like asking to be put to death. However, little did I know that cantering is her best, most easily controlled, most well behaved gait!
Basically, just trust him. He sounds like a trustworthy guy so let him be trustworthy.
And RELAX! I like to pretend that I'm riding fancy dressage and I'm just going along, talking in a very stereotypically British accent, being very posh, sitting up straight, letting my legs (in their fancy dressage breeches, I have an overactive imagination...hahaha) be very long, etc.
Another thing I like to do a lot (maybe for when you're more comfortable cantering) is I like to start out hand galloping Lacey (I 2-point while galloping which helps me feel more secure), then bring her down to nice relaxed canter. That way, I feel like I'm going so slow once I'm cantering and I'm in so much control that I have no issue cantering in a relaxed way.
You can do it! I bet he'll be so good and you'll be like "I LOVE THIS HORSE!!!" :)
ETA: I'm not sure if I'd have him canter on the lunge line the first time, unless it's one of those "you're going in a straight line, instructor is holding the lunge line and walking/running so s/he stays in a straight line with you" things. Especially since you say that he's still figuring out where his feet are, he's going to have a much easier time balancing with a rider (and therefore be less potentially scary) at the canter going in a straight line. You can always try it, but that's just my $0.02. :)
My favorite way to get a canter is ask coming around the corner of the arena, then canter the long side (or the short side if you have a really large arena) and slow to a trot before the corner. That way I get a short "safe" feeling canter and a for sure stopping place. :)