Thank you for all your comments.
Ok, the dog wasn't necessarily a 'dangerous dog', but it was clearly out of control and running headlong into 'traffic'. Once he caught up with the dog and saw what had happened, the owner admitted responsibility, and added "I have no idea why he keeps running off like that"! I have to say, given that kind of history, if I was the owner I'd be inclined to keep it on a lead!
Please don't be too harsh on my son (who is 22 btw, and not a young child!

) - he is very new to commuting, and has only been riding his bike to work for 2 weeks, so lacks the experience we all gain over time (and after a few knocks) to take more care.
Having said that, every situation is different, and he assures me that he was riding intelligently (albeit at some speed) and was mindful of others using the path. The dog came out of nowhere, galloping free, and there was nothing he could do. Had he been going slower, it would have made little difference - hitting a large dog is likely to knock you off your bike!
We are obviously concerned about the poor dog, but as contact details weren't exchanged, I doubt we will find out how it is.
Mini cheddars were normal cheese flavour, not Marmite flavour - I didn't know you could get those - they sound yummy!!