Another thing that can help (and you can set this up with friends out on walks if they are willing) is to have a handful of treats, call him and as you call him, throw the treats on the floor, and click as he gets to you. That way he will be encouraged to look for the treats on the floor rather than jumping up. If you can then practise with as many people as possible (giving them handfuls of treats in advance, obviously), he should start to associate going to people with needing to look on the floor for the treats.
Work at this in tangent with teaching him to have his feet on the floor.
If he has been ignoring "down" for a while then I would probably choose a new word (we say "floor") and start right from scratch, setting him up to succeed as much as possible.
When praising him by petting him, do really slow, calming strokes on him rather than ruffling him or anything as slow stroking will help to calm him down.
You'll get there in the end, don't give up!