My doc says cos I'm allergic to ondansetron that I'm also allergic to antihistamines, what can I use for gritty itchy eyes due to spring allergies?

Huh? Ondansetron allergy is very uncommon. I am no allergist, but there is no apparent overlap between the antihistamine H1 and H2 receptor and the antiemetic HT3 receptor that I am aware of. In addition, antihistamines have been used as part of the treatment of ondansteron allergy. I suggest you clear this up with your doc.
Highly unlikely. I think it is time for you to consult an allergist to help you sort out this issue. That you are allergic to all antihistamines and ordansetron is a highly unlikely event. You will also need to distinguish between side effects vs allergies.