540-590 HP for a champion with a sub-40% winrate, when HP wasn't even his problem and other non-tank champions still have higher hp. 50 HP is less than 150 gold worth of HP. If you actually thought this would help him out at all, you're totally delusional.
When a champion has a <40% winrate, that's the time to be heavy-handed with buffs. Nunu hates everything about this season. The runes, the item changes, everything.
With him being so abysmally useless, now is the time to experiment with his kit. Add additional affects to his abilities and make him more supportive like Ivern.