Welcome, I would say tidymix with no sunflower or peanuts and I use to offer a bowl of different veg every day to get her use to it. Bits of fruit like apples grapes should be given lightly or kept for treats for training purposes.
Oh my goodness he is adorable! Love how cute baby greys are
Not able to help too much on the food front as your little guy is a lot younger than my boy was when i got him. My baby grey is 6 months old now and on a harrisons pellet food with a bit of high quality seed mixed in (no sunflower) and he has fresh fruit and veggies daily. But like i say my boy is 6 months so a bit older than your baby!
Hope he brings you all the happiness my birds have brought me!
Roughly chopped fruit and veg for our larger birds during the day. Also a no sunflower and no peanut parrot mix e.g. AS30. Scarletts sell it and it is also available elsewhere. For supper, a finely chopped bean & pulse mix (with added finely chopped veg) goes down well with all of our birds (budgies, jardine, eclectus).
A good mix of colours, textures and flavours (that you can vary daily) will provide interest, and all the vits & mins that they need. Persevere with things that are not taken immediately - birds often come around to them eventually e.g. Jardine Jessie and Ekkie Frankie seem to be eating nearly all their fruit & veg these days, especially the greens, and they are less interested in the dry food.
I'd recommend regular weigh-ins for a young bird, preferably first thing in the morning, after first poop and before food, which is quite easy if you use cage covers, or maintain a dark room until it's wake-up time. We use a homemade T-stand on a kitchen scale that measures in 1g intervals.
I also suggest removing the mirror from the cage as some birds can get attached to their own image and become less inclined to bond with their human carers.