I'm glad things are going well with your big boy! It sounds you are up to the challenge!
Sometimes other dogs who growl are unfriendly, sometimes it's your own dog who annoys the other dog because they don't have great social skills and the other dog is just trying to let him know when he's crossed the line. My dogs play with each other all the time but with other dogs less frequently. They get told off by other dogs when they are being too... whatever. Chase is a major butt sniffing fool and some other dogs don't appreciate the intense scrutiny. As long as it's confined to a growl type correction, not an all out fight, it may just be the other dog letting yours know his behavior is not appreciated. I don't usually mind if my dogs get those types of corrections from other dogs, I figure it helps them learn how to interact appropriately. I will certainly step in and correct behavior I'm afraid is getting annoying, but if the other dog corrects him first, that's OK. And my dogs end up looking at the other dog like, "What the heck, Dude?" How do you tell them it's their own fault?
I hope your obedience classes go well. I've been to plenty of classes where there have been adult dogs who never had training before and the trainers seem to keep things under control. You may be getting a little bit of a late start, but since they claim labs settle down some between 2-3 years of age, maybe you'll find he catches on quickly and is the star of the class! Good luck!