Londongirl's advice is good.
Inadvertently we have a similar challenge that we're starting to turn to our advantage.
Barnaby's WCS and 6 months old today. He spends every day with me in the office in town, so of course there have to be a few constitutional breaks to get him through the day. We have an established ritual of walking the same route several times a day and always stopping at a certain spot in the park to do business. Next to that spot is a kind of allotment area in which over the last couple of the owner has decided to house his flock of 10 hens. This was all very exciting for our boy, to the point of him several times failing to remember to do business at the appointed time and place.
For the last week we've turned tis excitement into a bit of field training. Now we approach calmly, ignoring the hens, do business, then quietly and calmly walk up to where they are rooting about then sit or better still lay still watching them. He's beginning to get very good at this, not barking or making noise, and indeed the hens are now happier to come right up to him (with a chicken-wire fence separating them. He stays on the lead the whole time of course.)
It seems to be transferrable skill too. Two days ago on our evening walk Barnaby was rooting about in a hedge but returned on the whistle. It turned out that there was a young pheasant in there that he could easily have grabbed, but contented himself with a thorough inspection.
Maybe on lead you can get your boy to do a little steady-practice as well?