Hi Lis,
First, I read on this forum that a fish can go a week or longer with out eating and usually be just fine, means there is still time to try to get this solved. I hope this will easy your worry a little bit.
Try crumbling some of his foods into smaller pieces, flakes,bloodworms, and try pre-soaking til soft all foods before feeding.
Do continue to try to find the freeze dried brine shrimp, call ahead to the stores to find who has it in stock.
Are any of the foods that you are feeding the same as what he was being fed at the store where you purchased him?
As suggested in a prior post, did you get the results of your tank water parameters= ammonia,PH, nitrites, nitrates.
Is there any possibility that he has now or could have had any thing lodged in his mouth or further back?
Is there any possibility that he might have gotten any thing in his mouth that might have injured his mouth even if he spit it right out?
Might explain the white stuff you at one point were seeing and his not eating. Injury that got infected.
Were there any pieces of gravel that were in his bowl that were small enough to fit in his mouth? What is the texture of this gravel.
Ref. constipation, you could try feeding him some small pieces of a pea, wont hurt him if hes not constipated. Thing is, he has to eat it for it to do any good. Take one frozen pea, completely thaw, remove and discard outer skin, only feed in side portion, pinch off small piece, squish between fingers,and feed either by placing in his bowl, between your fingers, or with a tooth pick carefully.
You could also do the following,I know I would, remove all the gravel from his bowl, and leave it out for a couple of days, watch for evidence of fish manure. If you should find some, it would rule out constipation, and indicate that he is getting some food in.
As was suggested in an earlier post, did you get the results of his bowl water parameters= ammonia,PH, nitrite, nitrate.
When you are able to deal with the frozen brine shrimp, also try frozen daphnia, the daphnia I feed is very very tiny. Very easy for them to get down.
And last but not least, try to PM bettababy.
Hang in there, and don't give up.