How would i stop a custom mob from moving until after a few seconds?

You could give it a really, really high level of slowness. 

...Or, if you want to make it a little bit fancier, you could try somehow saving the locked entity's position as NBT data and repeatedly teleporting them there, or try constantly overriding their motion vector to zero. (Maybe just overriding x/z motion to zero, unless you also want them to levitate while frozen.)