Counting Kicks

I'm 23 weeks and just a week ago my baby boy was kicking like crazy. This week its seemed to slow down. Every once and a while I will see how long it takes him to get his ten kicks in (I've read that they have up to two hours) and he usually get it within twenty mins. Is it normal for him to slow down like this? This is my first pregnancy so I notice everything and everything worries me! Advice please! Thank you ladies!