The female is picking on his him and that is stressing him out, stress out a reptile and their bodies produce more emzynes that encourages more reproduction on coccidia. Coccidia in low levels is thought to be a normal part of a bearded dragons digestive system and not harmful in minimal amounts.
But, when they become stress or otherwise ill, their immune system that keeps the coccidia in balance goes crazy, producing emzynes that makes the coccidia mass reproduce and running the dragon down more, creating mre stress.
Remove the stress and get a fecal to the Vet, even if he does not have coccidia, the stress alone is running him down and making an opening for other complications.
Make sure he is in a basking area several hours a day between 95-100F, he needs this heat for his system to digest food properly and to boost his own immune system.
You can also give him some watered down pedialite or gatorade to rehydrate him fast. use an eyedropper and place some on the tip of his nose, if he will not drink that, lower the side of his lip gently and place a few drops there, once he has a taste, most will usually start licking...... Ours favorite flavor is Strawberry-kiwi, none of them refuse it.
Soak him also to get fluids back in him, you can even soak him in the pedialite. rinse at the end of the soaking