Well, teaching an older dog is not really any different to teaching a puppy, inasmuch as you have to be very very methodical about it.
The mistake most people make is to get a good recall in the house/garden and in low distraction situations on walks and then assume the dog "knows" the recall in all situations.
The difference between teaching puppies and older dogs is usually that with the older dog, the cue has been "poisoned" in the sense that it doesn't see the cue as "come NOW whatever you're doing" because that cue hasn't been taught methodically enough, generalised and proofed.
If I were you I'd teach it from scratch, using a different cue (a whistle is good - it's ear catching and always sounds the same no matter what), start in a low distraction environment (house) then working up to slightly higher, then low distraction outside, then slightly higher etc, always setting the dog up to succeed and NEVER using the special recall signal unless you'd bet a hundred quid he will respond!
In between, you can use the old recall cue. So if you're not willing to bet that money, use the old cue, then if he DOES come back, as he's running to you then use the new one and feed something special. Make the new one irresistible. Use steak if you have to. Save up bacon rinds, bits of roast chicken etc. What you're aiming for is a dog who ultimately always comes back when he hears the special cue - because that's what he's always done. He does it without thinking. Like a soldier who automatically ducks when he hears a bang. A conditioned response.
If you've already tried a whistle and poisoned it, try a different type/tone.