It sounds to me as if, unfortunately your poor Betta has multiple illnesses.
How big of a tank is he kept in? Does it have a filter and heater? Do you change the water weekly - if so, how much water do you change? Do you put any chemicals in your water like dechlorinator? How long have you had the tank?
If his dorsal fin is gone that is fin rot, it's a bacterial infection brought on by poor water conditions and stress to the fish. The best thing to cure this is to keep his water clean. So do a large water change when you get the chance.
The other illness isn't as clear to me. Does it look cottony like a fungus??
My suggestion is get a picture, quickly make an account on a website called Photobucket, there you can upload the picture and take it's IMG code (Looks like [IMG*]...blah..blah...[*/IMG] And paste it here and hopefully we can identify it for you and help you get your fish better :0
By the way, welcome to the forum.