Definitely get with a lactation consultant as soon as possible. I'd avoid the pediatrician unless yours is breastfeeding friendly and knowledgeable...most of them in my opinion aren't.
I don't know what the issue is, but I have an oversupply problem and both my kids right around the 4-6 week mark did the same thing. I just had a ton of milk and they weren't mature enough to deal with it.
Here is a great link describing it.
http://www.kellymom.com/bf/supply/fast-letdown.html
My sons' behavior was exactly what the link described.
What I found helpful was to let my sons latch on in the upright position (as if they were standing on your lap). The gravity helped them swallow. I had to do this for a few weeks.
Does your son do this when he is asleep/sleepy? That is one clue. If he does it primarily during the day when he is awake that might be an oversupply indication. when they are actively sucking, they have too much milk, whereas at night, they tend to be sleepy and suck less vigorously and so don't have that issue.
I wish you luck. Don't wait to long. This is an important time to get BF established.
Take care.