You're welcome! 
Well, to me it sounds like he could be somewhere on the spectrum now, but it seems more or less like he is in denial about it and simply never really learned to cope with the issues that come with autism.
Maybe his mood swings actually come from being sensory overloaded from too much input from his environment (sound, smell, a conversation to pay attention to) or maybe it's even from being emotionally overloaded in a way. Emotions can be a really confusing thing and if he is in love as well, he may find it difficult to deal with the chaos that comes with it.
Or he just has severe anger management issues (caution advised in this case). Unfortunately, it will be a hard if not impossible task to tell that one without knowing him, so it will have to be up to you to observe that.
Since you're aware of the allergy he has, you might want to propose him a backup plan if it's a somewhat severe case.
As I've mentioned before, spontaneous changes in routine, schedule and plans isn't exactly something most of us are comfortable with, so in such a special situation it could be worth talking about that first and not just out of the blue while you're already there. 
In case he does get an allergic reaction of some sort when you are there, it might be better to fall back to a lesser activity. The allergy could be tiring for him and an additional source of overwhelming input. As such it could be better to do something relaxing. Getting some ice cream (and maybe coffee and cake?) certainly sounds like a good idea for a fall-back activity. This would sound like a suitable moment to propose a new plan to make up for the "failed" zoo date, should things really go bad. The aquarium visit skibum proposed sounds like a good example of what could be done instead.
Good luck on Friday!
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Diagnosed with Aspergers.
BSP-errors are awesome.