From what you say this may be a dominance but also equally could be plain bad manners b/c nobody showed him proper etiquette (mainly other dogs). But you too can help stop this by making it clear that he doesn't get to use his body to control other dogs. My puppy is very physical with some people, who encourage jumping by petting and cooing. She knows better with me, I turn and ignore her or walk away. She also does it with our 4yo dog now that she's in adolescence, which Harley is too. Some breeds more than others are very physical with the way they go through life unless redirected.
She does NOT do it at dog class even though the other dogs are all 10-20 pounds and she's 52 pounds. She self-handicaps herself by lowering her body to their level and playing very gently with these little dogs. They actually get going so fast on the rubber floor that she'll run to me a few times because she's too heavy to go that fast without slipping on the floor. So she's not bodily controlling them because she knows not to after being disciplined by other puppies in other classes.
My 4yo still self-handicaps for the puppy so she does not hate being tackled though her puckered lips show she'd like a little more room she does turn toward puppy and re-engage. To encourage more room I trained an "excuse me" which I click at the first turn of her head toward me. Both dogs come for a treat and a little cool off before playing more.
The way to help guide your dog is by teaching a leave-it or excuse me which means break off now and use a clicker with good meaty treats. Leave it works for deer, elk, dogs, and people barking off my deck and has saved countless things I've dropped. You can train that for other dogs too and if you reward and are consistent you'll see progress. What you don't want is for him to get physical with the "wrong" dog and get hurt.
Consider too that this may be some misdirected herding behavior and his way of blowing off steam or frustration. Maybe you could set up play dates with safe known dogs or find some local outlets for his herding instincts. I have several friends that drive a couple of hours a few times a month to places that have sheep and teach green dogs. Since he's a Sheltie more more more exercise. Maybe buy some small agility equipment and teach him agility, get a rally book and work privately with him, teach tricks, free shape some of the incredible things you see on TV or YouTube like hopping in a box or stacking bowls, or freestyle is always fun for some. Get him a job and by the time he reaches full adulthood in another year he'll be more confident and lose the brat hopefully sooner.
I sympathize with not being near a trainer, I almost lost my mind when I moved looking for one.
Maybe he's just a teenage guy looking for fun and getting in trouble out of boredom or lack of mental/physical outlets. I know when I was a teenager boredome spelled trouble. :dogrolleyes:
I've recommended Emma Parson's Click to Calm, Patricia McConnell's Fiesty Fido, Karen Pryor's Don't Shoot the Dog, and Leslie McDevitt's Control Unleashed to several people with similar problems b/c they were very helpful with my similar issues. For you Click to Calm and Control Unleashed would be top of the list. CU is more of an intense read but continues to be my favorite dog book of all time. Parson's book is excellent with explanation, detail, and reasons things work, and timing.