First of all, don't stress. Such a monster needs time to adapt to a new setting. Do not feed him for a couple of days. You are only stressing him.
let it go hungry.
On the rat, leave it for once a month. Aros are not highly eqquiped for the digestion of mammal protein.
The squid is okay ( although it spoils the water ), as is okay other strips of white fish, preferably fw fish.
I have tried also, with very good results, cubes of raw chicken ( you have to wash it very well in running water to take out that "gooish" stuff ). My monsters love it.
But foremost, be calm, and don't stress it. if it is healthy it will eat, even if it takes a lot of time ( and it looks as it has enough reserves with all that food it got at the previous owner ).
How about some pix?