If he is really good at what he does and you want to keep him obviously, it is a little more challenging. You must have a frank conversation with him about this

If he is really good at what he does and you want to keep him obviously, it is a little more challenging. You must have a frank conversation with him about this as soon as possible. May be you can arrange a quick call with him and say something like hey I noticed your time logs might have discrepancies. Just wanted to talk about it and make sure we are on the same page. Then ask him clearly Is this a mistake because I was just wondering about this item x that I did not have the specs ready for but your time logs shows an entry for a day earlier etc. I understand that mistakes like this can happen but I am trying to track our efforts into this product and want to keep as realistic as possible. Let me know if you have any questions. Then let him say what he thinks about it. If he was just trying to test you, he will realize that you are smarter than that and he will probably stop. If he still does not stop this, that is a major red flag for what could come later for you. Good luck!!