Dogs humping my dog

[IchBinAnnaOfCleves]

Just got back from a walk in the local country park, massive pack of dogs with a couple of dog walkers and the biggest dog of them all (Great Dane sized but wasn’t a Great Dane) jumps on my docile male lab.
He’s then trying to get away and running between the nearest person’s legs (one of the dog walkers) to protect himself.
I’ve gone to retrieve him and then had to physically push the other dog away and said to the people that my dog really hates being jumped on.
They neither cared or did anything to get him so I just grabbed my other dog and walked back in the direction I’d come from.

I had a similar incident not long ago with another large dog who appeared with no owner in sight and just wouldn’t leave my lab alone. I was completely stuck as every time I walked away, the dog followed and was continually jumping on my dog’s back.

Are they trying to dominate him?

He’s very, very placid and it feels almost like he’s being bullied!

Are they any ways of stopping this happening to him?

Poor pup! It's less dominance related and more a play behaviour the dog hasn't been taught is unacceptable. It may be hormonal if the dog hasn't been neutered or spayed. If it's not nipped in the bud and the dog that's doing this behaviour isn't trained out of it, it can get out of hand and become a nuisance, or lead to fighting for dogs who hate that kind of rough play. It's a shame those dog walkers did nothing :( if they happened to be a business and had their van nearby, could could have a word with them about it.

For your dog, it's going to be tricky being the victim in this. If a dog off lead comes over, it's going to be the case of advocating for your dog and trying to distract the other dog and moving away from them. See if they would be more interested in a stick or old ball, throw that, get them away from you and hope the owners/dog walkers can be responsible.