What is this bump on my horse?

My horse is a 20 year old QH/Saddlbred, and he is a very loveable, happy horse. I am going to call the vet because I'm really worried, but I want to know if I should be expecting the worst:
   
He has this weird bump on his back, right on his spine. He doesn't like it when I push on it. It almost feels like there is fluid inside, but it's sort of hard too. It's kind of difficult to describe. I am terrified that it is a tumor.
   
Pictures:
   
http://i214.photobucket.com/albums/cc148/dakota_92_2007/bump011-1.jpg
   
http://i214.photobucket.com/albums/cc148/dakota_92_2007/bump002.jpg
   
Thank you for your time.
   
~Lizzy
   

Bot larvae can cause lumps like this on the back and there will be a grub inside the swelling so the vet will need to lance it and remove the grub before you clean and protect the wound.
   
Otherwise, it may be an abcess. If its hot and painful and imflamed it is probably an abcess and will need veterinary treatment since it is on the horse's back.
   
The best advice here will be from a vet.
   
Ps. Dont ride him either if the saddle will press on his back onto the lump or you might cause damage.

Thank you. I have a pretty good idea of what caused it now and he is on the road to recovery. :)