Have you got his/her cage ready yet? Do you know if it is a he or she? Babies can be a little clumsy so you might want to get some rope perches to help him/her grip better and maybe set them a little lower than normal to start with so he doesn't have so far to fall. Natural branches are good too to exercise the feet and help blunt those baby needle sharp claws. If you can't place the cage against a wall then cover the back with a sheet.
Even if the breeder says you can let him out straight away, I wouldn't. I'd let him get used to the cage for a day or two and get into a routine (know when to expect food etc.) and check out the surroundings beyond the cage. I let Kobe out the same day because the breeder told me to - big mistake - he took off and panicked because he didn't know where to land. He eventually flew into the wall and landed on top of the radiator (which was off), but somehow got a leg stuck in the vent slats. I had to run to get his weight off the leg and try to get the leg out. All I could think was "OMG - broken leg!" and when calmer, "what went in must come out" - eventually I freed the leg - not a good start, and certainly not a way to gain trust. It was a couple of days before I let him out again and this time all was well.
Kobe was weaned at around 4 - 5 months old, but did regress a little. I didn't feed him formula, but I hand fed him all his food. I should have stopped as early as possible, because even now, 14.5 years later, Kobe eats best if I sit in a chair right next to him. He whines and whines if I don't. So beware! Kobe was a little picky about food, but I found he loved peas, so I stuck peas on top of anything new I wanted him to try.
Pionus can freeze if frightened. The baby Kobe did that once because of a shadow of a bird that flew past. You can't snap them out of it. Just sit quietly with them (if used to humans) until they relax again - I remember it took quite a while (maybe as much as 20 minutes) for Kobe to move.
Remember a new bird has lost his old flock and everything he knew so will likely be very quiet to begin with as in the wild it is a dangerous situation to be in. So I would just give him space to settle in quietly and give him the same food he is used to, to begin with so that there isn't too much change all at once. But as well as the seed the breeder gave me, I also gave Kobe a bowl of fresh fruit and veggies too. I can't remember if he ate anything that first day, apart from the chopped fruit I took for his trip back on the train in a birdie rucksack. I also grew sprouted grains, legumes and seeds for him from day 1 and even now those are still his favourite fresh food... as well as pomegranite arils, blueberries and grapes. He also likes squash (raw and cooked) and goes mad for the raw seeds straight out of the squash - I freeze them so he can have them year round. He also loves scrambled egg.
Move smoothly and calmly around your new baby... no fast, jerky movements that could frighten him. You want to aim to keep his body language as relaxed as possible. Pairing your presence with favourite foods will also help gain his trust. Being around you = good things happen! So he will want to be around you more.