take him out the tank for as long as you can (a full day and night would be good). you can also move stuff around. every betta i had and put fish in after him he attacked, but fish then him and had about 80% sucess rate. not all bettas will allow you to put other fish with them. i like corys i keep mine with soft river rock and there whiskers are fine. shrimp (cherry, ghost), apple snails, neon tetras would be good there easy on bio load id say 10 would be ok.
barney-blue & red crowntail-20g long
berry-red, white & blue halfmoon-20g long
sunny-yellow & teal crowntail-2.5g
blue-blue & peach halfmoon-5g