Please be advised that I am not a speech therapist. However based upon my limited experience, I would like to offer the following suggestions.
Being able to speak so as to be understood is a major part of being human. You want your son to be able to communicate as well as possible regardless of his grades. I would ask your son how he feels about speaking. Give him the chance to tell you or to write about his feelings. Spend some time with this sharing back and forth.
Even if your son is not uncomfortable, then I would begin by playing sound games with your son frequently but briefly. Keep it fun, light, and varied. These ideas are listed in order of importance from greatest to least.
1. Read and practice MouthSounds, the book and CD, by Fred Newman. This book teaches sound effects that will work those speech muscles, and he probably won't mind learning to make cool popping and clicking sounds. The book also explains how we make sounds.
2. Does your son have less difficulty when he sings? If you don't know, find out by having him sing something along with the radio. It's not uncommon for children to be able to sing clearly and yet not speak clearly. If singing helps, then sing or chant everyday.
3. Practice tongue twisters or alliterative sentences one sound at a time. For instance, work on the "l" sound one day. "Leaping lizards like lollipops." You can find many databases of tongue twisters online. This practice also shows your son that everyone can have difficulty speaking certain letter combinations.
4. Imitate the voices of television anchors or celebrities by listening to a clip and then trying to recreate the sounds and rhythm.
My understanding is that in public and private schools speech therapists have long waiting lists because there aren't enough to handle the number of children who need help. As a home schooler, you don't have to wait for a speech therapist. Start immediately with MouthSounds and singing. If you don't hear any improvement after 30 days of consistent practice, then you should look for a specialist to rule out hearing or neurological issues and to pinpoint what will help your son.