I know I shouldn't worry but I do!

Cristina • 2 years of trying to conceive. 2 Angel babies. Welcomed baby boy 06/01/15.
My little guy is now 4 1/2 months and he still doesn't giggle or roll over. We do tummy time but he has only learned to roll to his side. We also tickle him and be silly and no giggles just smiles and this high pitch squeal from his throat like he wants to laugh but doesn't know how. Any others experiencing this?