You could try an APDT Trainer.
Or if you’re going to go the DIY route….
Does the dog respond to his name? If not, teach him, if so:
Start in a secure environment with minimal distractions e.g.living room.
Have a handfull of tasty treats, cheese, ham, sausages, liver cake.
Say dog’s name, when he gives eye contact, say “Come” (or here or whatever you choose), show him treat if necessary, and lure him towards you/to follow you.
Click and treat (or just treat if that has no meaning to you).
Repeat this until proofed in the secure environment.
Add distractions – now try it outside in garden.
Proof the recall in this environment.
Add more distractions… at this point you might want to use a training line.
I would recommend training classes, and/or try a good training book e.g. Pamela Dennison’s “Complete Idiot’s Guide to Positive Dog Training.”
Be patient, have fun and try to enjoy it!