I had a similar situation with my guppies, so I'd recommend first getting a breeder's net. Then add some new fish into the mix. Maybe a school of tetras, or rasboras. After that, re-introduce him. If that doesn't work, you may have to get a new tank.
10 Gallon Tank:
~Countless Pond Snails~1 Espe's Rasbora~1 Harlequin Rasbora~
5 and 5.5 Gallon Tanks:
Avatar by Fenghuang
Many of my scaly friends from my original tank from 2011 have passed, except for two! I am in the process of re-doing my tank. :)