Hi Jacks Mum, welcome to the forum! I bet your real excited about bringing your boy home. You will find tons of information here, so just take your time and look around some of the various rooms such as the food room, etc. Keep in mind that Greys as with all birds should get a varied diet of fresh fruits, veggies, pellets, nuts and some seeds as treats. Honestly I would ask the breeder what he/she is feeding him on and start out there though.
I have a question for you if I may..... You said in the quote above that your worried that THE perch in the cage might be too big for him. I don't know if this is a typo or what but it is always a good idea to have several different types of perches for your guy to stand on. Perches of all shapes, sizes and materials. Such as rope, smooth wood, rough wood, sandy perches (for trimming those nails and for him to fine tune his beak on), etc.
Anyway, I welcome you again and hope to see you around the forum and look forward to seeing some pictures of your new guy once you get him home!