NOW, when I asked is he sneezing while he's swimming.  Answer:  NO,  WHAT?  have you ever seen a duck go swimming and they always like to dunk there heads in the water and splash it out, squirting water every where to try and clean out there nostrils but this little guy isn't doing that.  HUM.
 
SOOO At this point what I would do, is take his happy butt, take a Qtip wet it, and clean the inside of his nose, Loosening up all the gooy stuff that's in there.   THEN put him in the Bath tub with some NICE warm Water, Let him swim in there, You may have to change the water a couple of times because he will probably turn the water Brown.  LOL
 
Let him Try to get as CLEAN as he can.  Clear out those nostrils.  Let him do his Bobbing thing and go to TOWN..  See if he can Unclog himself or get anything that may be in those nostrils out..  Once he is done try squeezing the checks to see if anything come shooting out. 
 
If so, then he'll need his tylan orally for a few days.