Has your daughter participated in hearing screenings at school? I would have her consulted by an audiologist/pediatric audiologist just to rule out any hearing difficulty that might contribute to her speaking loudly. Once we rule out a hearing difficulty, then we can consider an SLP to work with her on proper vocal hygiene and protecting her voice if that is needed. I would say let's rule out a hearing concern first.
I would recommend you take her to a speech clinic to complete a comprehensive speech and language assessment. Please be prepared to provide detail to the professionals there regarding your concerns. When you say she does not express herself clearly, are you referring to telegraphic speech? Do you mean she has jumbled thoughts and nonsensical statements? When you refer to lack of coordination, are you referring to oral motor movements? Is she groping with her mouth and saying different sounds for the same word when she tries to repeat herself?
To start, you can help by taking her to a speech language pathologist for an assessment. You can encourage her to speak with you and use choices, gestures, drawing, etc., all kinds of ways to communicate to keep her interested in expressing herself. We want her to associate communication with positivity so she does not try to shut down or stop talking. Model statements for her. For example, if she is saying "bird come down sit" and you realize she is talking about her pet parakeet, join her in conversation while expanding the sentence such as "Yes, the bird is sitting. See him perched by his nest?" Engage her in conversation and continue to shape the anatomy of a conversation, taking turns, looking in each others' eyes, smiling...these pragmatic abilities enhance the charisma of a strong communicator. We can still help her communication at home using these techniques while we wait for an evaluation from the SLP.
Please note I am a medical speech language pathologist. We are required to complete a Bachelor's, Master's, 9-month externship, and then apply for certification. While we are trained specialist within the domain of medicine and healthcare, we are not physicians. I am not a doctor.
Regarding the hearing loss, is it conductive or sensorineural hearing loss? Because a lot of the sounds we imitate come from what we hear, hearing difficulty can impact how he is interpreting the sounds and what he says subsequently. Has he been to an audiologist yet for a follow up? My recommendation to you at this time is for him to see a pediatric audiologist and potential speech language pathologist for an evaluation and aural rehabilitation, if appropriate.
I would expect more than 50% of words to be intelligible in phrase and simple sentence form for someone half your sons age. Without an evaluation from a speech language pathologist, there are many answers to your question. An immediate word of advice is to offer smiling and turn-taking so he is not discouraged from speaking. We want him to be excited about trying to express himself and communicate with others without feeling like every time he opens his mouth something is wrong. By no means am I implying this is happening now, I'm just opening with that word of caution.
Next, we want to naturally slow him down by encouraging him to emphasize the endings of all his words. Try makiNG iT a gaMe where he spends more time at the enD of aLL his WordS. It is a technique I have used with students or patients who were dysfluent and cluttered (speaking very quickly unawares). Like I said, there are several things that I have a hunch could be occurring. It would be inappropriate to offer a diagnosis or strict answer without an evaluation. Please take him to the MD for a referral or go directly to a speech clinic for an evaluation. Feel free to keep me posted if you found this at all helpful!
Now, the beautiful thing about brain plasticity and speech language pathology services is that there is no age restriction! When I was in graduate school, it was drilled into our heads "there is no known cause nor cure for stuttering." What we have found in research is that some individuals have been found to be more fluent when working with auditory feedback or singing. There is definitely something he can do. I encourage you and your husband to find a Board Certified in Fluency Speech Language Pathologist in your area and get an evaluation and initiate some services. Being a great communicator is not necessarily about being 100% fluent; whom among us really is? I have had a few fillers and interjections (i.e., uh, er, etc.) myself in the midst of conversation! So, he can work on both presenting himself as a strong communicator as well as techniques to smooth the "bumpy" speech that may occur during higher verbal demand situations.
There is not just one cause and the course of treatment varies. Some individuals can see a speech language pathologist while others need to visit an ENT (ear nose throat specialist). I would say first, if the kids are interested in addressing their puberphonia (which for some kids might be habitual depending on how they identify), they can see a speech language pathologist that specializes in voice and/or an ENT and start with an evaluation.