Baby Is Not Grasping

My son will be 18 weeks (roughly 4 and a half months) this Sunday, but unfortunately this baby is not grasping anything yet. He puts his hands to his mouth, but he doesn't bat at toys or try to grab anything.

My husband and I will put things in his hands and he doesn't seem to notice. His development in all other areas is either right on or ahead. Should I be concerned? What can I do help improve his development? Any comments would be most appreciated.