Hmm. Did you already click on the stone with the hole to examine it, so that he knows it's too heavy?
Double check that you've not missed anything from this sequence:
Click twice on the fount for Feodor to check the pipes there.
Now a main intake is seen at back of the fountain. Click main intake so that Feodor can follow that to the front part of the fountain under the floor.
Click on the paving stones and Feodor removes the mosaic stones to show the pipes.
It leads to the left side. The large stone slab over the pipe is too heavy but it has a hole in the middle. Use the improvised grappling hook on the hole of the slab.
"Bleat, Watson -- unmitigated bleat!" ~ Sherlock Holmes