Is he depressed and lethargic?
Drop the spinach, as it contains an anti-calcium chemical which is normally not a problem. Add baked winter squash. Acorn, butternut, yams, etc., have lots of good slow and fast carbs and fiber. He may just need more carbs. Since he's so very skinny you may need to force feed him the squash, even though force feeding is only a last resort.
Does the inside of his mouth look nice and pink, or is it too pale? When you gently palpate his innards, do you feel big lump or bumps (although you may be able to feel normal stomach, liver, and intestine).
Is he shedding normally? Was his last shed more than 6 months ago?
If you're not 100% positive he's a male, he may be a she and have shrunken retained eggs.