Hello John and welcome to the family, glad you could join us and a good warm hello to Alfie. I am so glad to hear you have rescued this grey, it sounds like he has had it a little rough over there, but in the right hands he should blossom out into a wonderful companion.
I am always proud of those who rescue greys for they get additional baggage that the ones of us who get babies don't. And by that I mean it's diet is not what it should be and the handling and care are not up to standards. But with your love and proper nutrition and care, he should be healthier and his feathers will be in better shape.
If you are working in a pet store, then you should have an idea of what is proper diet for a grey, but if in doubt, we have a bird food room with lots of threads dealing with what they should be eating and not eating. Browse thru and read for lots of good information and on the whole site for that matter.
Weight can vary and 400 grams is certainly well within the boundaries, but the feel of the keel bone is the best indicator of under or over nourishment. You should feel it but it should not be prominent. If he hasn't seen an avian vet yet then that is in order to determine his health and you can get it sexed as well.
If you have a photo of Alfie you would like to post, we would love to see him, even if he is not at his best, for it will give us a peek into what you have now and later you can post another of when he is better and healthier.
Keep us informed of Alfie's progress, we care about your new friend and hope he turns into your lifelong companion.