M.K.
L.,
As a 28-year perinatal instructor and lactation specialist, sometimes the resolve to this issue is as simple as ...
Trying different kinds of nipples till you "hit" on the winner that Nathan can comfortably and easily get his tongue underneath. Many babies have a very strong tongue-thrusting reflex, even while at the breast.
You might consider giving him either a "Nuk" or "Mam" pacifier to practice on. These two pacifiers are great training tools for the baby with an intense tongue thrust.
Hang in there ... he'll be graduating from high school before you know it!
Have a Magical and Meaningful New Year,
~M.
http://M.-writeon.blogspot.com
http://melaniekissell.com