Hi there - another Montrealer!! Where abouts are you living?
A few possibilities come to me:
1. I agree, there's a good chance it's gas. How's your latch? Can you hear any noises when he sucks - like clicking? Depending where you are, a lot of the CLSCs have nurses trained to check your latch - you could drop by. If his latch is poor, he'll take in a lot more air.
2. It could be a supply issue. How's his weight gain and diaper count? Does your milk supply seem to be good? Sometimes when the flow is low, they get impatient and pop off a lot. You're also at the 6 week growth spurt time - did this just start? He might be getting frustrated because he's just hungrier! If this is the case, it should get better in a couple of days.
3. Overactive letdown. Sometimes if your letdown is really powerful, they sputter and come off, squirming too - they just have a hard time controlling the flow. Do you ever spray at these times? If this is the case, there are also options to help.
4. Reflux. You might want to check the Colic, Crying and Reflux board, just to see if any of those signs ring a bell.
Let me know if any of this seems to fit or if you have more questions.