Has he said that he he finds it embarrassing and that he wants to improve this area of himself?
I only ask because its hard to help people if they don’t want the help.
It would be useful for him to look back and identify when he does it most; i.e if its when he is in a hyper state and talking rapidly or if its a constant occurrence maybe there is a working memory problem or is there other underlying condition affecting him.
The only way to get idioms right is to look up there meanings and sit there and practice using them which you could do with him I suppose. But he could know them perfectly and still have a problem using them in certain situations because of the ADHD.