Hi and welcome to the forums..
A few questions, as being vertical can be caused by a few different things.
What size is the tank?
What is your cleaning schedule? And what % do you take out each time?
Do you use conditioner?
Is the tank heated? What is the temp?
To me it sounds as if possibly he is cold.. is there anything else going on with him like bloating or changing color? Fins look okay?
Also, is it happening just once in a while or is he like that all of the time? Could be he is also napping, and when they do sometimes their tails dip and they look like statues..