I think i'll just severely limit his durability, heat resistance, speed and strength instead. The magical resistance ties is with his backstory, and the regeneration is an offshoot of his main ability, which i wouldn't want to change - polymorphy.

The reason why i stressed durability so much is because he has no weapons other than his body, so if he wouldn't be able to harden it enough, Ijok wouldn't be able to use any form of bladed weapon.

His weakspot couldn't be more obvious, as his entire body, save for the weakspot, which glows with an orange light, is transparent and clear. Plus, it's bleeding magical energy, so for anybody who can detect it, it would be an easy guess that it's the main source of his power.

And again, fireballs and the like will work just fine. It's only magic that does not have any physical carrier, being purely energy based, that he can resist.