Try this to make your baby more comfortable and less scared of this position.  Carry and burp your baby up close to your shoulder so that his head is above your shoulder and facing the opposite way you are.  This is an easy way for your baby to get used to pressure on his belly side while being in a comforting position.  It also engages his neck muscles.  Make sure to support his head if he still does not have control of it.  As he gets stronger, you can move him higher and higher above your shoulder.  Aim to hold him in this position at least 15min.  This will count as 1 round of your 4 daily sessions of tummy time.  Feel free to repeat this exercise.