It's not fatigue. He claims that he could pitch on three days rest all through the season. Apparently he's a throwback and has a rubber arm. Although, personally, it looks like he's lost about 2 mph on his fastbasll since the first month.
Anyway, ya, he's just not the ace he looked like early on. Probably a 3.80-4.00 ERA kind of pitcher. Six innings and 3 ER feels like a pretty typical start for him, with a gem or two thrown in. And man does he give up HR's.
If I owned him in a fantasy league I would never start him at GABP, but would relish a start in San Diego and LA. A lot of the HR's he gives up barely go out.