If it’s any comfort there are hundreds of spaniel owners up and down the country who own a dog just like yours!
In fact it’s probably the commonest fault of all in spaniels and the only way you can stop it developing is to insist on the dog working close to you from the outset.
This is taught in early training.
Spaniels are crafty little so-and-sos and soon learn how far they can pull away before you ‘spoil their party’ by blowing the stop whistle.
They also know at what distance you lose control, or take your eyes off them.
Once they reach that point of no return they will be off doing their own thing, deaf to whistle and voice commands. If your dog is still fairly young there’s a good chance you can get it back on the straight and narrow so to speak by returning to basic training.
Cast him off and make sure that he responds to the turn whistle and comes back into you in order to be cast out to hunt in the opposite direction.
Keep the pattern tight (between 10-15 yards) and don’t let him go beyond the limit that you feel comfortable with, and in control of.
If possible you should train your gun dogs in the company of other dogs so that he not only learns to work in a group but to also keep his attention on you and your commands.
If he’s an older dog and thoroughly set in his ways then it would be best to keep him out of the beating line and just use him for picking up instead.