Can thumb sucking cause problems with speach?

My 2 and a half yr old step son had a physical done last friday and the doc said he has to take speach therapy cles. I kinda figured that the doc would say that, cuz i have known him since february and his speach is not how it should be for his age at 2 and a half, compaired to his almost 4 yr old brother who speaks as well as a kindergartener. anyway was wondering if maybe his thumb sucking habit is causing him to have a speach problem! You should hear how he says balloon, his tongue sticks out when he says the loon part!