You can try a few things:
Try getting a 2 gallon+ tank, he might want more room to swim around.
Get him a heater and a thermometer to monitor his water temp. the water he's in now is a bit cold, he should perk up with warmer water.
Get him silk plants (plastic can really tear him up) and some places to hide and explore. Those floating Betta logs can be great, though sometimes they take awhile for the fish to get used to them.
Try a Marimo Moss Ball, PetCo and PetSmart sell them. PetSmart is cheaper but they are still like $7, a little pricey but I have 3 of them and my fish love them.
Use some API Stress Coat+, it'll help alot in making him feel better. He could be stressed out a little and unhappy with his tank.
For food you can try Aqueon Betta Pellets (i love those), or New Life Spectrum Betta pellets/ Small Fish Formula. Try getting him some frozen Bloodworms or frozen Brine shrimp, my fish go nuts for those, but don't feed either of them to your fish more then acouple times a week.
I hope some of that helps out.