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.
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