Hi Ofer,
The fact that you found him in a rock might be the key piece to the
puzzle. It's another indication that yours is not Ocythoe
tuberculata - and that is actually good news! You weren't getting any
direct answers about feeding because nobody really knows much about
the needs of a pelagic octopus since they haven't been studied very
much.
Since yours is apparently a benthic octopus (like the vast majority of
octos) it can probably do just fine on the same sort of food given to
other 'typical' pet octos, namely crustaceans.
What you need to do is get a supply of appropriate sized live crabs,
shrimp, or amphipods and keep a few of them in the octo tank at all
times. Your octo will hunt them down and eat as needed and will also
get the opportunity to keep his brain active. Eventually you may be
able to feed frozen shrimp (thawed of course) but, in the beginning
especially, live is much better. In the wild, an octopus will eat just
about anything that represents a good meal for not too much effort so,
as he becomes more used to you, bigger, and hungrier you may be able
to add other things to his diet such as fish or squid but always try
to give him at least some live food.
As to what size is appropriate; I'm not clear whether your octo is 2
inches overall or has a two inch head. In any case, look for food
animals that are slightly smaller than the octo's head. Something to
keep in mind is that octos seem to lose interest in smaller food items
as they grow. As an example, where I volunteer, we generally keep a
Giant Pacific Octopus (GPO)(Enteroctopus dofleini). By the time one of
these fellows reaches about 10 pounds or so it has completely lost
interest in any food item smaller than about one pound - apparently,
smaller stuff just isn't worth bothering with.
Calorifically yours,
Alex