Okay, it sounds like you did a fine job of injecting the frozen Mysis and administering the Diamox orally and it is great to hear that your male is already doing much better. Well done!
I would continue the regimen of Diamox using the proper dosage, but just feeding your stallion two of the larger frozen Mysis that have been injected, or perhaps 3-4 of the smaller frozen Mysis that you have on hand each day.
And I would be prepared to perform a pouch flush on your male after he delivers his latest brood, which should be relatively easy after his sphincter muscle has been stretched out during the birthing process. That will assure that his pouch is cleansed very thoroughly this time, which may help to prevent any further problems with pouch gas.
But if he does develop similar problems with positive buoyancy and pouch emphysema during the course of his subsequent pregnancies, administering the Diamox orally via gut loaded or injected feeder shrimp is a good way to handle the situation.
In your case, Carol, I have an idea that the problems with gas bubble syndrome you have been experiencing are the result of transferring seahorses from from the hyposalinity maintained at the LFS to the higher specific gravity in your home aquarium, so hopefully adjusting the salinity downwards as you have done will protect your seahorses from further problems with GBS.
For future reference, however, you might want to search this forum for the terms preventing GBD or GBD prevention tips, which will bring up posts that explain other measures home hobbyists can take to help minimize future problems with gas bubble disease or gas bubble syndrome.
Best wishes with all your fishes, Carol.