Hi, I am an American Board Certified Neurologist and I am very glad to help you with your health concerns.
You have mentioned that he gets slurring of speech only during when he is tired, if this is correct then this is unlikely any TIA or ministrokes. If his speech problem is from tia then the speech problem should occur randomly, irrespective of whether he is tired or not, but in his case it seems the speech symptoms are occurring when he is tired so these do not appear to be TIAs.
The Plavix and Aspirin can be continued now as before and he does not need any change in his meds because he does not appear to be having more TIAs.
Regarding the TIA/Stroke workup, I am not sure what has already been done, I will give a list of the various tests that are necessary to find out the cause for the TIA, if you see any of the below mentioned tests have not already been done then he needs them, if they are done then there is no need to repeat them.
These are the various TIA/stroke related tests;
MR Angiogram of the brain
Blood tests for cholesterol, blood sugar, triglycerides etc
If any medical conditions like high blood pressure is present then it needs to be treated aggressively, and if there is any history of smoking and heavy drinking then these too need to be stopped or kept under control.
Once again I reassure you his speech symptoms do not appear to be TIA for the reasons discussed above.
Please let me know if you have any further questions; and if you feel you have received the answer to your question please rate me so that I can receive credit for the time and effort put by me in answering your question.