I would put it down to young horse getting nervous when faced with new things.
He's calm when in an environment that he's used to, but gets upset when in new places.
Firstly, check that he is not on a rising plane in regards to his weight/body score. As he has reached five,he may need a reduction in feed to compensate for his slow down in growing rate (like what Sugars Mum said

)
If this paddock is horse safe where the chooks are kept, then put him in there for a week or so and gradually move his feed bin closer to the chooks so he has the chance to gradually come to realise that they are not fire breathing dragons that are just out to eat him. As he overcomes his fear you could then start doing ground work exercises to challenge his strength of character and to confirm your leadership role, ie; if you can lunge him and drive him towards the scary chooks and then back the other way, or get him trotting over trot poles and gradually move them closer to the fiesty feathered fiercesome egg layers, then these exercises will help you when you encounter the next new thing to him and you can set about in the same method until he is listening to you and you can then ride on as normal.
Get his feet busy and his mind back on you...after all, you are the leader (aren't you???)
Hope this helps.
Deb
