Yes, it could easily be a few weeks, and there could even be some disappointments along the way because if he is still insulin resistant and by fasting his glucose drops lower than his body is used to, his liver will dump glucose to try and redress the balance, but when his pancreas secretes insulin to deal with the glucose, the liver (which should at that point stop producing glucose) doesn't get the message, so continues to produce it until either it gets the message to stop or its stores are depleted. Therefore, he may even see some slightly elevated levels. This should disappear once his body starts to recognise his new lower levels.