So there are a few theories floating around. The one is.
1. He did actually have a bone disease that was very visible (I don't really believe it based on the Viking culture as you mentioned)
2. I mentioned earlier that he fought with an acrobatic spinning style that made it appear he fought like he had no bones
3. He was impotent or suffered from erectile dysfunction.....hence boneless....though I don't know that he'd live with that joke of a title
4. He was actually Ivan the Hated. However in Latin 'hated" would be Exosus. The Latin word for "without bones" or "bones" would be translated as "Exos". It's possible some scribe messed it up or it got messed up in translation over the years. At least that's on wikipedia. "Ivar that Hated" would make a lot of sense from the English records. "although it is hard to align this theory with the direct translation of his name given in Norse sources"