DS is 3.6 and has been having trouble with a few sounds for a long time now. Most words are perfectly clear, and there are normal things that he still can't say (F instead of Th for example)
The sounds he's having trouble with are ones like bl, br, tr, pl etc (blue becomes flue, plum becomes fuh-lum etc)
We've been trying to help him with them, but he seems unable to make the proper noises.