What medicines should i not give my six-year-old grandson... what does not mix together.. thank you?

There are too many bad combinations to answer from such a vague post. You get the most from this site when you are specific. If you have a couple in mind provide their names and why you are tempted to use them. That gives us something to work with. Vague=no comment. You are welcome to start over.

You should not give your 6 year old grandson any medication without clearing meds with his GP or Pediatrician. Meds dosage is weight related. There are web sites that can check for interactions like https://www.webmd.com/interaction-checker, but list is too long for the limits of this site. And no web site knows your grandson like his Physician.