My year old avocado plants leaves are drying up. I raised my avocado tree from a pit. When it became larger we moved it from a window sill to our master bathroom. He gets sunlight through a block window over the bathtub. The bathroom stays warm and a bit humid. I give him diluted plant food about every week. What is wrong with it?
The avocado tree requires as much sun as you can give it, together with roots that stay moist but are well drained and nice gentle breezes to keep the leaves on the dry side. My guess is that while your bathroom may appear well lighted it is insufficient for the tree, and with the air being humid and still it probably hardly has time to lose moisture at all so the regular feedings will do more harm than good.
If he is living in a reasonably small pot you could take him outside into the sun and remove him from the pot to take a look at the roots. You should see nice light coloured roots on the surface of the root ball, but be very careful not to break apart the root system - they hate this disturbance.
Ideally he will have a much more lighted location, as bright as possible. In humid conditions cut back on the water and feed; being generous with water will tend to encourage rots and diseases.