Hi! Fish do get tumors, but I agree with ANHEL; it looks to me like he may have Lymphocystis.
Lymphocystis causes cauliflower-like and/or pimple-like growths. It's caused by a virus, and is one of the most common viral aquatic diseases. It's usually self-limiting, meaning that the fish's immune system can fight it off within several weeks or months. In other fish, it may be a chronic condition, meaning it persists over time.
Feed a quality diet, reduce stress, keep the water clean and warm. This will help the fish's immune system to remain strong, and hopefully, allow the fish to fight it off.
Here are two photos, followed by info links:
University of Florida - Lymphocystis Disease in Fish
Pet MD - Viral Infection (Lymphocystis) in Fish
Lymphocystis - Info and Cure