K.H.
Yes they all do it because they can , but it is the worst place for them to try and stand up , my youngest slipped and caught her eyebrow area on the tap and she had to have it glued back together (she was 17 months at the time). Get a non slip bath mat that sticks to the bottom of the bath so he can at least stand on that and has a less chance of sliding , that and stay by the bath all the time , until this phase passes you will just need to shorten bath time so that you don't end up sitting in the bathroom all evening!