Tom Arnold -- Dropping 90 Pounds Ain't So Sweet

Tom Arnold is half the man he used to be, dropping 90 pounds in 9 months -- but he tells TMZ, it wasn't easy ... he had to cut out sugar and bread from his diet and bust his ass exercising.

54-year-old Tom tells us his son Jax -- who was born in April -- is the inspiration for his weight loss. "My son is already cursed with an old dad, so he doesn't deserve to have a fat dad too."

According to Tom, the diet was a huge part, but he also did tons of circuit training, cardio, and took lots of Arnold Schwarzenegger's Muscle Pharm supplements.

Hasta la vista, belly.