Crohn's disease?

[weepootle]

Is this correct use of the apostrophe?

My husband had to write a letter including this and his boss corrected it and told him it can't have the apostrophe as it makes it personal to him (Crohn). However, it's named after him so it is personal to him surely?

That is the correct spelling of a medical condition. The disease is presumably called after the person who discovered it or something like that. NICE (national institute for health and clinical excellence) who write guidelines for the NHS call it so

guidance.nice.org.uk/CG/Wave22/2

But as to whether the medical profession has got the grammar right, I dunno!