Sorry to hear about Simba, it is really upsetting news. There are a few of us here that have dogs with elbow dysplasia. Deska was diagnosed around 3 years old, he pulled up lame after an agility comp and just didn't improve. By the time we got a diagnosis the arthritis had set in and was pretty bad. There had been other subtle indications before that but I kept getting told it was just behavioural.
There are 3 types of elbow dysplasia, Deska has fragmented coronoid process (FCP). He had the fragments removed when he was about 5 years old. You'll have a better idea of treatments when they know whether and what type of dysplasia, and also if there is any arthritis.
I do recommend seeing a physiotherapist and getting some exercises and a better idea of the types of exercise he can do. I'm afraid agility is probably off the cards. Any sort of jarring may make things worse. Deska continued doing flyball for a couple of years after dx, only on low heights but we stopped agility immediately. One time we did a fun agility course for retired dogs, where they put the jump bars on the ground, but Deska still tried to jump the jumps, I guess he was just too conditioned to jumping. So even if you put the jumps on the lowest height Simba may still try to jump at the level he knows.
As for rough and tumble games, not a great idea. But playing with his mates, esp if they are gentle like shelties, should be fine, just don't let him get bowled over. I can't see a big issue with running, as long as it isn't flat out. Sometimes it's got to be about quality of life.
Exercise is really, really important. You need to keep the muscles around those joints strong so it takes pressure off them. Deska still walks twice a day, in winter it's tougher and on longer walks I take a stroller so he can take rests. The other important thing is to keep Simba really trim, not least because you may have to carry him at times, but also the less pressure on the joints the better. I know that's hard to do when they first show signs of pain, because they don't want to move around. But once you have a diagnosis and they get him on some medication to settle down any flare up it will be easier.
I've done a few posts on managing Deska's arthritis, I'll do a search and post the links for you. Deska is now on daily NSAIDs, supplemented by pain killers in winter. But he's 10yrs old and can still chase down a bicycle at 30km/hr so don't despair too much.