A.J.
Anytime a difficult phase is happening, no matter where, you have to address it. Since he's 6, and knows under no uncertain terms it's not allowed, you need to enforce it a couple of times on a play date-hovering or not, and then he'll stop. It's like anything else, they only do it when you're on the phone, or in public, or on a play date a few times until they see it's not allowed.
Whatever you're going to do, make it very firm and enforceable in the moment. What's more egregious than being unkind to his friends and developing a personality that will make him unliked? Let's say you're going to make him sit away from his friends for 15 minutes while he goes and does a hard chore, and then needless to say, he's also lost some privileges that will need to be earned back as well as having the consequence-but the delayed removed TV is not enough to address the behavior in the moment.
Before the play date, warn him that you will be watching him, and the first time he is rude to a friend he will be removed from the group for x amount of time and will ....move the firewood-whatever (six is plenty old for hard chores-my 5 year old daughter helps move wood), and if he throws any trace of a fit, the consequence will increase. He will ALSO then have to behave for a week to earn back the privilege you gave him to start with-the TV. Be sure to explain to him exactly what being rude to his friends entails, so there is no confusion when you warn him.
Unfortunately, you really have to do it. Tell the other parents you're working through a thing and warn them too and ask them not to say, "Oh it's OK it's OK" etc. when you enforce his consequence. They will respect you taking charge.
Hover. Spy. When he does it, go give him one calm reminder that he's being bossy, speaking rudely, whatever, and he's about to lose TV for a week, and if you have to remind him again he will also leave his friends and do X just like you discussed. If he continues, remove him to do x. If he tries a fit, deal with that separately and firmly. You don't have to get flustered or mad. Bite louder than bark. At his age it shouldn't happen more than a couple more times because he'll believe your warning the next time.
This is occurring at this age because it hasn't been handled firmly. You're right the surly 'tude needs to go if he is going to be a well liked person. Don't bother rewarding behavior that should be expected in general, like politeness, other than the occasional, "I love you, I'm so proud to have a polite gentleman for a son" comment with hug, but of course praise him when he changes his behavior after your warning if it's something new he's learning.
It will help HUGELY if you are consistent at home when he displays a bad attitude for any reason, even if there is no sharing situation, that way he knows how to change the attitude quickly at a warning. If you crack down at other times, the play dates will get much better.
I wouldn't recommend removing play dates, he needs practice and consequences to get this right. Just firm up. Time outs and removed privileges are sort of "empty voids where consequences should be" which is why he still has the attitude. Sure, he should have to earn back a lost privilege like TV after he has proven the 'tude is better, not have privileges "just because" when he's got a 'tude, but he should also have an immediate negative effect, like a hard chore if he makes a wrong choice despite your warning. This is good for his character as well as being a good consequence, and you can get some help in the process.
Good work addressing this. My kids don't always have to share either-with each other, but they sure do with friends and they are not allowed to be mean. They were firmly disciplined at the first attempts of that. They go to school with a few of those "My kids don't have to share" types and all I can say is..ew. We avoid them. Its very frustrating to us parents who actually taught our kids how to be nice, and now they're getting bossed around by rude kids who think their toys are so precious.