This is slight delay.but sometimes it could b poasible.we can wait for month only If at the time if birth he was not admitted in Nursery no history of seizures.than we can wait for another month.sometimes it slight delay normally in many kids

Around this time they should sit without support. Does he sit with support.? All children do not crawl  in the same way.  How does he move about?. There seems to be slight delay.is there any tightness in hip joint? ( tight when trying to wear diaper? ) Or is his legs like a rag doll.? Does he move and lift his legs when lying down ? Does he transfer objects from one hand to another? Either you can wait one more month to watch for the milestones or show your pediatrician to make things clear.