Have you spoken to his brother about this? Maybe a quiet chat about your concerns. I'm sure that if he's finding life hard (which I suspect he is), then he would be glad to know that he has support from you. However these bruises have been caused, it's highlighted a potential problem that needs to be dealt with before anything more serious happens.
Now is the time, if he isn't already, for your uncle with Downs to be getting help himself. He is, himself, a vulnerable adult and his future without his mother needs to be considered.
You could perhaps begin by you and your 'normal' uncle having a chat with the family GP. He or she can help you to find a way of encouraging your other uncle to get out into the community. Perhaps a day centre or a social club one afternoon a week, where the activities distract him from the stress of being out of the house.
Also, have you contacted the Downs Syndrome Association -
http://www.downs-syndrome.org.uk/DSA_lstLitera ture.aspx
They have a number of fact sheets to download that may helpful.