Personally I advise you stay in an enclosed area for the first few times. You need to practice transitioning back down to the trot. If you do it in a large space and your horse takes off.. that won't be so good.
With young, green, or re-trained horses, you want to make things easy for them. If you give them too much space, they're overwhelmed. If you take too much space, they go into flight mode. So ride in a small that is big enough for the horse to feel comfortable.
Cue him off of voice when you canter in the beginning, and only do it for a few strides at first. Make sure not to do any straight lines, stay on an arc. I like to canter into and out of corners and trot on the straight parts.
Once he understands your voice cue, then you can start placing your legs into position, then saying canter.. then slowly work on doing it off of leg alone.
Don't forget to half halt before you ask for the canter. I had my green horse take a nose dive and I nearly got flung off as well. Not a good experience!
The transition from lunge line to under saddle will be alright. Just remember it won't be as smooth as on the line because of the added weight and the need to balance you and him.
One last thing. Make sure you ask for the canter when your horse is paying attention and slow at the trot. If you ask when your horse is geared up, you will create a bad running-into-canter habit. Trust me.. it's ugly and hard to deal with.
"Strength is the ability to use a muscle without tension"