Is my baby gaining enough?

I'm concerned about my eight and a half week old son. He was 9lbs, 3oz at birth, and had been gaining weight fairly steadily up until the last week and half or so. He has been sitting at 12lbs, 2oz now for almost eight days. He has plenty of wet diapers, and approximately 2 dirty diapers a day. He is exclusively fed, but I'm concerned that my milk supply may be dropping due to stress. I've never had a ton of milk, but lately even when I'm pumping after each feeding there is almost nothing there. His stool is bright yellow, as it has been since birth, but I am noticing that it seems a bit more mucousy than usual, and is lacking the regular little "curds" that it used to have. He is alert, and usually fairly happy and smiling. He gets a bit fussy at night, but isn't screaming his lungs out. He also sleeps through the night (has since four weeks old) so he doesn't get any nighttime feedings (between 10pm and 7am). Should I be concerned? He started out in the 95th percentile for weight, and is now dropping to somewhere around the 50th.