I'm a CF who will start a job working with this population very soon. That being said I don't know what I can bring to the table, but I am curious to see what more seasoned folks have to say.

Developmental disability or not, your client still has a form of dysarthria. What compensatory strategies you choose to target are going to depend on what speech characteristics are causing the decrease in intelligibility. Is your client's decrease in intelligibility because of things like imprecise articulation, or poor breath control, or deficits in prosody (monotone, slow rate)? What ever the cause of the break down, you could totally target compensatory strategies using drill based exercises. Maybe practice functional words or phrases using these strategies. The main compensatory strategies I remember from grad school are slowing rate of speech and over articulating. But once again what strategies you choose to implement are largely going to depend on what your clients deficits are.

If your client is able to communicate verbally, I would be very hesitant to pursue AAC. But that's just me. While AAC seems like the go-to for CP, the fact that your client already feels alienated because of their speech would make me hesitant to pursue AAC devices or options. Unless this is what the client in his family wanted.

That's my two cents don't know what good it does! Once again curious what other folks with more experience have to say. :)