Hi N.. My first son also had severe speech difficulties and we used a variety of resources for him. If your son is four, he's too old for ECI, but he may qualify for free services through your local public school. I personally did not find those services too helpful, though, because they were in a group, only a couple of times per week for very short sessions, and they were not allowed to do oral-motor exercises with the kids.
The University of Houston has a speech clinic that was somewhat helpful. The speech students run a pre-school type program under the supervision of their professors, and the cost is very reasonable. Each child gets an individual session every day as well, that you can observe through one-way glass.
The most helpful option was also the most expensive one, of course: private speech therapy. We saw Ellen Carlin in The Woodlands and were very happy with her for the most part. She did a thorough evaluation, then designed a program for my son's specific issues, using lots of exercises to target the facial muscles involved. She even designed a program for my second son who couldn't suck well enough to breastfeed when he was born.
I don't know of any DVDs (you might do a google search), but it would probably be helpful for you to make games out of things that involve speech skills with your son as much as possible. Blow bubbles. Use straws for drinking. Lick peanut butter and other stuff off a spoon. Fill your cheeks up with air or water and then blow it out. Make funny faces at each other. See how far your tongue can reach in different directions. Pretend you are monkeys and give him - yes- lip massages, where you have one finger on the outside and your thumb on the inside of his lip, and gently squeeze and roll as you go around the whole perimeter. He doesn't have to know that this is for any reason but silliness!
Best wishes!