Big bird babies are naturally clumbsy but they need to be allowed/encouraged to perch to strengthen their legs and feet, something you can do to help them out is make sure you have large diameter perches so they don't have to grip so tightly and to have different textures of perches available. If your bird seems like he falls a lot then I'd do several things. To keep him from injuring himself in the fall I'd place towels or a folded blanket underneath where you have him perching and that way should he fall its softer to land on.
Something that you can do to help him with improving his strength and his balance is to allow him to sit on your arm and do short little movements of your arm so that he has to spread his wings to help maintain his balance. Nothing too drastic and not in rapid succession. Move your arm up and down and side to side so he learns to correct his balance from all different directions same as with how the wind would wiggle a tree limb etc. This does several things it improves their balance, it exercises their wings, feet and legs.
If you plan on trying to harness train him, NOW is a great time to get him used to wearing a harness, it will help because he's so young he won't know that it isn't normal to have you putting one on and for him to be wearing it. Young birds are much more adaptable to things like that than trying to train an older bird to a harness. There's sure to be a lot less blood letting (yours

) by starting them off young. And if you get in the habit of always putting the harness on before you take him outside it won't be a big deal after a while. Just help them by equating or placing a word on the harness so he learns that to "go outside" or "go bye bye" "Go for a walk" means he has to have his harness on. And if being outside is an enjoyable experience for him then he won't fight you so much to put the harness on.