I have a friend who apparently never learned to make the 'th' sound, and just replaces it with the 'd' sound or at the end of words with the 'tch' sound. He run

I have a friend who apparently never learned to make the 'th' sound, and just replaces it with the 'd' sound or at the end of words with the 'tch' sound. He runs his own business and does moderately well for himself, but I tend to find that speech habit very distracting and can only assume many of the people he does business with are the same. I've considered bringing it up to him as something he could improve on, but I haven't figured out a way to tell him that wouldn't sound horribly condescending.