Aggressive dog - advice please.

I am looking for advice from experienced dog owners please.

This weekend we took our 14 month old Patterdale, Baz, to the caravan.  We have a seasonal pitch and he has been coming with us since he was a few months old.

As we were going to be busy putting up the awning we tied him on the long lead so he could be outside with us.  He was happy chewing his stick until we heard him barking and snarling and spun around to find him jumping at a little boy.  The child was frozen with fear so we dashed over and pulled the dog away.

The boy started to cry so I comforted him and looked at where he was rubbing his arm.  There was no mark at all (it wasn't even red) but he told me that the dog had bitten him.  I put Baz in his cage in the car and went to see the boy's father (who was sitting outside his caravan nearby) and explained what had happened and apologised.  The man was fine about it and told the little lad that he shouldn't go near strange dogs.

I was absolutely mortified though.  Baz is a bit of a handful and can be boisterous and excitable but he has never shown any sort of aggression towards anyone before.  We don't worry about letting him off the lead on walks and the only reason he was on the cable that day was because there is livestock in the field adjacent to the site and we worry that he will get through the fence.

We wonder whether it was the fact that he felt vulnerable tied up with someone running towards him that caused him to jump.

Does anyone have any suggestions because I am so worried that this might not just be a one off?