First thing to instal is the stop whistle. Make sure this is 100% irrelevent of distractions (dummys, smells, claps, bangs etc).
Once this is perfected, when he puts something up (rabbit, pheasant, skylark - whatever) blow the stop whistle.
At this point he should be fairly close to you, so dont blow it too hard (loudness of whistle depends on urgency of the command). He should stop immediately.
If not - nows the fun bit
You have to run out and get him...dont make this a game - you have to mean business.
Take him forceably by the scruff (do not hit him or shout at him) back to the sp