Hi S. - this is absolutely normal - he is just exploring and discovering - to him it is just another part of his body. I would just let him do it and maybe even tell him that's his bottom then move on to his leg, foot, toes, etc.... making it fun. If you make an issue out of it, he will sense that and either shut down or want to do this all the time. Keep it simple with him - it's just another body part.
If this continues out of the bath I would watch it - if he is overly curious then of course it becomes a hygiene issue and then you can just talk with him, on his level about his fingers getting poopy - yukky, etc and limit only to the bath. This is actually a good thing as it is a physical pre-cursor to his potty training abilities of holding on, letting go and he will know this place and how it feels due to his natural exploring.
Many, many children do this and especially those who are tactile, hands-on learners and those children that are very curious and high thinkers - they explore all things constantly - it's totally normal.
Alli