Thanks for the great reply. It was really helpful. It was air in the pouch. I could see it in there when I looked closely. I think it may have gotten in there when I had him out of the water briefly to transfer him a few days ago.
I think I got all of the air out today but it was tricky. He’s too small for the bobby pin or even a small blunt syringe needle tip to fit into the opening of the pouch so I tried a slightly different approach. I took a small, fine but blunt tipped syringe and squirted salt water at the opening of the pouch while pressing the needle tip against but not into the opening. This fully inflated the pouch. I then held him upright, gently squeezed the pouch, and massaged the pouch opening with the needle tip. This made him vacate the pouch. I repeated that 3 times. The seahorse did not make it easy though. He was a feisty little stallion and fought me every step of the way. He even bit me twice which I didn’t think was possible for a seahorse.
He is still a little subdued but he is swimming right side up again.