I wish I knew something to recommend to you that would be different from what your son's speech therapist has probably already recommended. But I feel your despair and want to mention a few things that might not have been considered.
As you live in San Antonio and your son's name is Xavier, I assume that he is growing up in a bilingual home. Although this does not routinely cause language problems, it does sometimes. So I would choose one language and use it with him consistently. And the language you choose for home should be the one used in his school. Read little stories to him and let him point to pictures of objects he recognizes. And always speak slowly and distinctly to him.
Since you say he is in special education, he must already have had some sort of diagnostic evaluation. Programs for children with disabilities usually require an extensive developmental evaluation prior to accepting a child. If such an evaluation has been done, someone on the staff should have spent time interpreting the results for you. Since a preschool for children with disabilities was arranged for him, the evaluation must have shown a broader pattern of delay than speech alone. There are other hints of that in your question, such as your mention of the fact that he was a little slow in developing eye-hand skills, needed for holding a cup. School personnel and the speech therapist can give you ideas to help your son develop. Your acceptance and love of him will be critical to his progress.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.