Definitely follow everyone's advice on getting an EI evaluation. My DS is 22 months and I had him evaluated last month and our first speech therapy aptmt is next week. I'm really looking forward to the 6 months of 1:1 (for 1 hr) that he'll be getting (after that he'd have to requalify but I think he'll be caught up by then).
What I wanted to ask is, does he use a pacifier a lot, or did he? Or perhaps is a big thumb sucker? The pacifier is the main culprit in our case, and after being told to limit it to only nap/bedtime and times of distress (crying, traveling) or new situations that could make him feel stressed... they suggested that we (1) make sure he only eats snacks one piece at a time. Is your son a "stuffer"? As in, a handful of goldfish goes in all at once? If so, single piece chewing enhances the mouth muscles more, so try that. Also, make him use a straw instead of a sippy cup as often as possible. There are some straw-style sippy cups out there that are great for no-spill comfort. Again, the object is to get his mouth muscles to move differently. Lastly, just play LOTS of "imitate my sound" games and focus on what different animals say (moo, baa, bleet, chirp, tweet, woof, bow-wow, meow, cluck, hisss, etc.). That makes the mouth move in all different ways.
These are all things you can do before the EI eval and will possibly have an immediate impact. I noticed they did with my DS and he sounds a lot like your little guy. Of course, every child is different, but I really think EI is the answer (or at least a step toward a solution) in this case. Good luck!