I hardly think it is inappropriate for a father & son to shower or bathe together at any age. It’s not a sexual thing. It’s a bonding thing. Curiosity about dads body is quite normal and it’s better to handle it calmly and in a relaxed atmosphere so that as he ages he will feel comfortable talking to his dad about any subject such as sex and safe sex when he hits puberty.
I think that boys of that age should not take showers or bath with their dads it don’t look right.