Hello there,
Bryn's behaviour can be for many many different reasons and you will neer know why - you can have an idea if you study his body language carefully, one book that helped me is :
http://www.amazon.co.uk/Talking-Terms-Dogs-Calming-Signals/dp/1929242360
This book is fabulous and very helpful when you are working with a rescue- I bought it when I got my little one who like Bryn was very fearful and unsure of men.
Most likely he has had bad experiences in his past but I worked really hard with all members of my family in getting him used to them!
We started off with non-confrontational body language so never approach him head on or lean over him, back up in reverse and let Bryn sniff them first and if he doesn't want to approach then respect his desision. Have the men and older children have treats to give Bryn but start off with them not looking at him sitting on the floor and just throwing the treats toward his area so he can assosiate them with rewards.
Again if he backs away or givees any signalls that he is uncomfortabel then respect his desision!
Younger children will be more difficult as they are unpredictable and can't understand the reasons for his behaviour so I would make sure that Bryn always has a safe place to go (like a crate) and that the chldren are tolld very firmly that they are Not to approach im while he is in there.
They should also know not to chase him or lean towards him or be too rough with him.
Who knows what this little guy has been through in the past but what I will say is that even though my little man had neer seen a child before in his life (and at the time I had a neice of 7 and a nephew of 2 that he was introduced to) the Cavalier in him took over and he was so perfect around them!!
Men took a few months to combat but now he is fine with most and will even approach some for cuddles!!