I doubt that's an abscess. They just don't usually look like that. That appears to be more of a tumor like mass. Which would mean you would need to find a vet experienced with rabbits to assess it and possibly surgically remove it. But even if it is an abscess it needs further treatment than just antibiotics.
http://www.medirabbit.com/EN/Cancer/Sarcoma/Fibro/Fibros_en.htm (tumor info)
http://www.medirabbit.com/EN/Skin_diseases/Bacterial/Abscess/skin_abscess.htm (abscess info)
http://wabbitwiki.com/wiki/Philippines#Rabbit_veterinarians (rabbit vet list Philippines )
There's also a facebook page that has rabbit vet info called 'Rabbit Awareness Philippines '.
Please do not just give your rabbit the cephalexin. Cephalexin is only safe to give to rabbits by injection. If not given in the proper way, you risk making your rabbit very sick. And if that is a tumor, antibiotics aren't going to help at all