They do not put MSG in sushi.
However, was the salmon.... raw... or smoked? Smoked salmon, looks raw.. but is like the salmon that would be for loxs and bagels. And is a cured salmon. Usually cold smoked and with a brine.
Or maybe the mayonnaise in the California roll? is causing a problem?
Or maybe he is just gassy... ? have you tried Mylicon Infant Gas drops?
Discomfort/gassiness, makes it hard for a baby to nurse.
Do you burp him?
But, traditionally, no. Sushi does not have MSG added to it.
The rice, is often seasoned... with rice vinegar in some places. And sometimes seasoned with "Mirin", which is a sweet sake.
Or maybe, the sushi was spoiled?
But you said you are not concerned about the quality of sushi at the restaurant you went to.
Or maybe it is the rice? Some people are allergic to rice.
Or maybe the Nori? The seaweed you are reacting to.
http://www.sushifaq.com/questions/sushi-msg.htm
The seafood, could be affecting your baby.
Babies, should not have seafood, until after 1 year old, I think. Some say after 6 months.
Try looking it up online... about seafood and breastfeeding.