Dear, don't worry. All kids are not same. Some kids take their time for achieving their growth.

Don't force the baby. Try doing tummy time for a few minutes during the day. If he doesn't lie down on the stomach then also it is fine

Hey please go through this articles

<span style="color:#3B5998;"><b><a href="http://app.babychakra.com/article/2203">Tummy Time for Babies 4 Months+: DIY Button Activity</a></b></span>

<span style="color:#3B5998;"><b><a href="http://app.babychakra.com/article/4400">Tummy Time Benefits</a></b></span>