Good evening There is no need to worry now ,like you said he's trying to sit up.Dont stress about this one if his other developmental milestones are as per the age. You can wait for now.Get him examined at his 6 month visit if you are doubtful. Take care.

Generally Rolling over comes at 2-5 months of age.   Proper neck holding comes at 3-5 months of age. Commomly baby start to sit with support at 5-7 months of age and  Sit without support at 6-8 months of age  Baby start to crawl at 7-9 months of age  Baby start to stand with support at 10-11 months of age  And stand without support at 11-13 months of age  Baby start to walk with support at 12-14 months of age and walk without support at 14 -15 months of age.  Wait for some more days  Encourage ur baby for each and every activities of ur baby  Stay with ur baby only  It will help  for  early achievement of millstones. 

Children usually start to roll over from the fifth month only. So do not worry Hello him roll over by showing his favourite toy to one side while he is lying on his back and try to stimulate him to roll over.