Our eldest cavalier (male unneutured) has started to submit to large dogs when we meet them outwalking. He has been well socialised and has seen a variety of dogs at classes, he doesn't do this when we meet our friends who have a huge lab.....after going straight up to the dogs and laying on his belly then rolling over he then barks at them as if he is wanting to play???

Is he frightened of large dogs or is this normal behaviour?

It looks really odd I have never seen a dog behave like that before

He is letting them know he is no threat and wants to play.  This behaviour is designed to appease other dogs, which is a better option than challenging them, even if it is bluff. As you said the jumping up and barking after is an invite to play most likely depending on his body posture.

Yes I think you are right. :-)

He wouldn't go up to them or bark etc at them afterwards if he was frightened would he??

Depends entirely on the dog - my oldest's first line of defense is offence, if she sees another dog and I'm not physically restraining her, her most likely course of action is to run full pelt straight at them barking, to try and frighten them off.  She's frightened of them though.

But in your dog's case it does sound like play. :-)

Our girl Mitz (SBT) always has done this, she runs up really low and when she reaches the dog lays on her back with her legs wide apart wagging her tail!! :D
looks quite funny and im happy she is submissive rather then overbearing for new dogs she doesnt know! :)

Rach,

Thats exactly what he does ;-)

My boy (GR) does this too !! :)
He loves to play and greets all dogs like this no matter what size!!
He's not frightened at all :)

Jiggy