hI David
I don't think another dog would help at this stage, imho.
Try to set up lots of situations where he is totally ignored if he jumps up. Get a good Sit, and walk around the park, and practice a Sit near to people after approaching head -on, as if you were going to say "hallo".
A good training class will do "Sit to Greet" exercises, where the dog is put in different situations and is only rewarded for acceptable behavour. One exercise which works very well is to tie the dog up and get different people to approach, only reward when dog is 4 feet on ground, or in a Sit or whatever you feel you want to aim for.
Obviously this should only be done if the dog loves people and is happy to be approached by them.
Just an idea anyway......I think the key is to involve as many different people as you can, and as many "strangers" as you can.
I tend to use "Off" if I need a word, as imho "down" can suggest to the dog you want it to lie down

(Well, maybe that's not such a bad idea.....:D )
Lindsay