I have a similar client at the moment. I am puzzled as to how to treat it. I am currently trying some education regarding nasal sounds/oral sounds and getting him to feel the difference by placing his fingers on other peoples noses. Beyond this, I am stumped.
It has been suggested to me to try getting the /l/ in the middle of lots of vowels sounds t encourage /l/ production. I will also try exaggerated tongue placement on the lips. Howvwer, I still feel he will nasalize.