If you don't want to hold a metal post in place until the concrete sets up, consider building a brace. Metal fence posts can be braced with wood boards to keep them vertical while pouring concrete. Brace the post before you begin pouring so you won't have to worry about it moving. The brace needs to be removable and strong enough to support the post on all sides. A simple brace design needs to support the post while allowing the concrete to pour in around it. You may need help from an assistant.
Set the metal post in the hole. Ask someone to hold the post upright.
Lay a 2-by-4-inch board (3 feet long) on each side of the post. Turn the boards so the 4-inch sides face the post.
Lay two 2-by-4s (each 18 inches long) perpendicular over the two previously placed boards. Place each of these boards on each side of the post with the 2-inch sides facing the post. Nail these posts down to the underlying boards, using 3-inch fencing nails and the hammer. Use two nails at each of the four intersections.
Lay a 2-by-4 (3 feet long) at each end of the previously placed 3-foot-long boards. Lay them down so their 4-inch sides face the ground. Slide them in about 6 inches beneath the ends of the previously placed 3-foot-long boards. Nail the boards together with two 3-inch fencing nails at each of the four intersections.
Level and centre the post in the hole, using the bubble level on the side and top of the post. Move the post and the frame around the post until the bubble is centred. The brace will now hold the post without assistance.
Pour the concrete, then wait for it to cure. Lift up the brace along the length of the metal post until it clears the top.