You're talking about an articulation problem. Can he make the sound of every letter in the alphabet? If not, it could be, as it was with my Pervasive Developmental Disorder (PDD) kid, a matter of oral motor function and very few SLPs even know how to do that evaluation. Check out this:
1. When he chews, is his tongue moving the food from one side of the mouth to the other?
2. Is the food being chewed in all of the teeth, on both sides?
3. The Speech Language Pathologist (SLP) should have looked at this but... can he move his tongue and lips to make each sound appropriately? Just go through the alphabet. Tongue has to be moved out for some sounds, lips change from round to normal position, etc.?
I would also very strongly suggest that the parents both go to a speech therapy session, that one half hour a week and see what the Speech Language Pathologist (SLP) is doing. Ask the Speech Language Pathologist (SLP) for homework they can do with him. I assure you that mine didn't get where he is today by leaving it to therapists. They have to practice the sounds with him every night for at least a half hour a night.
I've thought about the software and looked through the Edmark ones. But I don't think software is really what you need. Think I'd focus on figuring out which sounds, age appropriate of course, he needs to work on and then work on those. eg something like a Dr. Suess book of the particular sound he's working on would be good for him to read aloud to parents. A children's librarian at the public library would be able to point out books for the sounds that he's working on. Also think about limericks for that sound, Sally sells sea shells... though of course, blends such as sh and th are not age appropriate.
Other thing that comes to mind is music (CD, tape) that focuses on speech. I'll dig up some names for you.
FYI: if he can make each individual sound but can't put them in the correct order, can't get them to work together, that's a whole other subject.
Check out asha.org. There are some good articles in there.