Boneitis is a deadly disease that cripples the bones, ultimately resulting in death. That Guy, who was from the 20th century, was stricken with the disease.
Assuming the same terminology exists in the 31st century, Boneitis means 'inflamed bones'; 'itis' meaning 'inflammation' and 'bone' meaning 'of the bones'.
Boneitis seems to be an extreme and accelerated form of rheumatism. If so, it could be treated easily with a number of disease-modifying antirheumatic drugs in combination with anti-inflammatories (like NSAIDs); both of which, ironically, exist in the 21st century. This would mitigate That Guy's need for a trip to the 31st century in the first place. On the other hand, if it is a dystrophic disorder as it is portrayed, there is currently no known cure - in which case his trip would be legitimized.