@Clairdelune - Sorry your buddy isn't feeling well. Cancer tumors on a dog's spleen can be either cancerous or not. There are blood blisters on the tumor and it may break and cause internal bleeding.
Some of the symptoms are pale gums, lack of energy, abdominal pain, and a low temperature.
It's best to take him to the vet. She will take blood tests, x-rays and check for internal bleeding. Spleen cancer is difficult to treat, but it can be successful.