How old is he?
Start adding distractions slowly. For instance, have your fiance play with him with his favorite toy and then you call him. You can try standing with your fiance while he plays with the dog, then say, "Come!" and run away. He should be enticed to chase after you. From here, you could have someone bring a dog(to your home) and let them associate on-leash. Give the command, and if he doesn't, bring a treat to his nose and lure him to you(a very short distance--a stride or two maybe).
From here, advance to letting them play off-leash at your home. Stand close to them and give the command, then run away.
Continue to slowly add distractions, and only add distractions as he masters the first ones. In other words, don't bring in the dog until he's perfect with coming to you as your fiance is distracting him. Don't unleash them until he responds perfect with him and the other dog on-leash. Taking him straight to the off-leash dog park will not do anything for him, since he is not at all ready for it yet.
Also, it'd be good to buy a 20-50 foot lead so that you can still work with him but know that he is not going to get too far away. One thing that I like to work on is having the dog in a stay, going a distance away, and having a distraction(a person and dog) seated about halfway between. The dog will be tempted to visit them and become distracted in mid-return, but continue working just as described before. You can have them sit a little ways off to the side so he is less distracted and have them ease there way closer as he improves. Hope this helps, and if you have any questions feel free to ask!
Good luck!