Using a whistle.
The whistle will give you an additional tool to use with your dog’s re-call; it is an as well as, not an instead of. The whistle can never lie to him, if you blow it, you must give a food treat and praise for the response.
When giving him his main meals, prepare his food and as you place the bowl on to the floor and your dog is about to eat, blow the whistle 3 times, short, but loud blasts. This will start to build up the association of the sound of the whistle meaning “ah food”.
Occasionally during the day, (5-6 at a time, spread out during the day in batches of 3), when you are close to your dog and you have his absolute attention and a treat in your hand, (always using high value such as cheese, hot-dog or chopped, cooked chicken), say his name and blow the whistle, peep, peep, peep and immediately give him the treat and a sincere, “good boy”. (Note; there is no obedience involved in this; it is not necessary for him to be in a sit, you need to reward/praise the reaction to the whistle).
Note. Do not rush to implement this when out on a walk or to test the theory, it takes a lot of patience. If you transfer the sound of the whistle to outside to soon, you will almost certainly train him to ignore it and therefore lose the positive effect it may have. I suggest that you use an Acme gun dog whistle, tuned and pitched for your specific breed of dog, available on line or any good pet shop. Buy more than one, they are easily lost and/or forgotten.
Progressing this exercise.
After a week or so, try calling him when he is at the other end of the hallway in line of sight, then out of sight. When he responds 100% of the time try when he is in the garden. When out take him to a contained dog walking area and if there are no dogs present, repeat the line of sight process. Keep practising at this level for a while, then, start to increase the distance and the distractions. Do not set him up to fail. Blow the whistle when out on a walk and he is next to you on the lead, this will help to increase his response when there are distractions such as people, traffic noise etc.