To me the key issue is his demeanor while at the zoo. If he is scared, crying, then I would seriously question the value in continuing to go....at barely 2 yrs old, he has time to grow more aware and have animal experiences in smaller doses first. IMO, honoring a fear isn't playing into it...fear is fear. A child can (and usually does) grow out of a fear, and become ok with the thing they used to be scared of....but in their own time, with support and baby steps...jmho.