First, don't give up hope! Miller is still pretty young and he's doing really well with a 90% recall
. (Wish Ra Kismet had been that good at that age)
It is frustrating when other dogs are more exciting and interesting than coming back to you and also, as you say, when recall is most needed.
If it's at a dog park where he developes 'selective hearing' do you immediately put him on his leash and head for home when he finally comes? Actually, is this what you do anywhere when he's playing with other dogs? If you do, then stop that for a while and make sure he comes to you and has a PLAY/TREAT first. Because if he thinks that by coming to you, and leaving all the fun time of playing with other dogs, means he's going on his leash and home, then he'll think twice over responding! He knows what you're going to do - stop his FUN!
"OMD she's acting like the fun police - I'm not going to hear that come".
He'll ignore you and keep playing because who wants to stop having fun!
It may take some time and of course, he may run off to the other dogs again, but keep rewarding and remember it takes time and patience to get a full total recall at all times
What you need to do is make recall TO YOU even MORE exciting than the other dogs. LOADS of HIGH VALUE treats and praise. I think you do use a clicker?????
Practice if possible, with a friend's dog, make a game of recall, calling him from the other dog to you and around to a friend etc. making sure he gets HIGH VALUE treats each time he comes. Not a leash in sight. He'll start to reason that, even if there's another dog, you've got the best rewards
"hey Mom's got a super special reward, beats even playing with other dogs, I'll run over to her now, right away, here I come Mom"
You really need to make yourself into an 'excitement machine', more so than all those doggy mates. Keep working with him he's doing so well! 90% is excellent!