Ivern should be treated as a ranged support/mage, he never wants to be in melee range and wants to focus on mage items like AP, mana, and CDR (CDR being the biggest since he is a support), but since he also has ranged basic attacks available he should reflect more ranged stats (which he has already minus the fact that he gets MR per level which no other range champion gets - pretty sure only Mega Gnar gets MR per level and not normal Gnar). I also would increase his ranged attacks to somewhere within 500-550 range.
Also his W is an on-hit effect, it should NEVER apply rylai's slow, make the game consistent on every champion and DO NOT have outliers that make no sense whatsoever to have. If you do this then Teemo might as well get Rylai's slow for his Toxic Shot and MR per level because it makes as much as sense Ivern (Teemo should have at least 550 range and not 500 in a more serious manner, he is an immobile marksman without a range increase spell like Kog'Maw gets - Move Quick isn't that good to justify 50 less range, also MF essentially has a Move Quick-like spell and has 550 range and very high burst damage.)
This turned into a bit of a rant. As far as feedback on Ivern, I need to play him as support on LIVE to see his actual power, I don't think he will quite work out as a jungler because a good jungler will counter him too hard (player vs champion, no specific champion vs champion aspects to this). But I will say that his health and mana cost on his passive should be reduced with armor and ability power that Ivern has because mages can clear the jungle super fast and take no damage and use no mana as they get stronger (or at least armor reducess the health cost, AP will do both to reduce the health and mana cost).