I would get a long line (those small clothes lines are perfect) and have that on him whenever you go out. If he heads for a chicken, give a correction (a jerk on the line) and tell him "leave it" or "No" or whatever you want to use as a command. When his attention gets back on you, praise, treat and reward. A few sessions of that and he'll be good. I wouldn't push it on the freedom though. Better to keep going with the line for a while than to have to start all over because he gets freedom too quickly and goes for a chicken again.