Do it in a neutral location. Don't force anything. He may never be friends with other dogs, but at the very least, he should be expected to remain calm, and ignore them.
What you need to do is don't immediately go to the most distracting place. Have your neighbor/friend have their dogs at the opposite end of the park (try and be the only ones with dogs there if possible). And then walk your dog towards them. The moment he stops focusing on you and tenses up when walking towards the other dogs, quickly turn around and walk the other way until you have his attention back on you and calm (have him do some tricks, make him really think about YOU, not other dogs). Then go back, and do the same thing, trying to move closer each time.
It might take quite a while... but it really does work, because you aren't just throwing him next to the other dogs and trying to scream at him to get his attention. You expect attention the whole time, and slowly add in the distractions, always retreating with the distractions until you solidly have his attention.
What you want is a dog that will not react at all at the sight of other dogs. He may never love other dogs, or play with other dogs... but if he will ignore other dogs, you can take him on walks and around other people and their dogs without the stress of him barking and carrying on.