You will want to do this in one go. Resist that. A photo wall goes up best when it goes up slowly, with him in the room, and when every frame is a small decision he gets to make rather than one you made for him.
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Pick the wall first
Ask him where she would have wanted them, then let him answer in his own time. Some fathers want the photos in the hallway where he passes them every morning. Others want them out of sight until they are ready. Either answer is right. Look at the wall with him at the time of day when the light comes through, because a spot that looks good at noon can be dark by four.
Check the wall before you commit. Tap along it with your knuckles. A solid sound means plaster or drywall over a stud. A hollow sound means you need wall anchors, which are rated for specific weights, so read the package. Run a stud finder across the spot you want. Mark each stud lightly in pencil.
Gather what you need
You need a pencil, a steel tape measure, a level, painter's tape, a stud finder, a drill with the right bit, a stepladder, and a helper to hold frames while you measure. Buy picture hooks or hanging wire rated above the weight of each frame, with some margin. Glass frames are heavier than they look. If you are unsure about a heavy frame, hang it on a stud or ask a hardware store clerk what the hook is rated for before you drive the nail.
Lay it out on the floor
Cut paper templates the size of each frame, or lay the frames face down on a clean sheet on the floor. Move them around until the arrangement is one you both like. Keep the largest frame near the center and the smallest at the edges. Take a photo of the layout with your phone so you can find it again if the paper gets moved.
Mark, drill, hang
Tape the template to the wall and mark the hook positions through the paper. Remove the paper, then measure again from the marks to the floor and the marks to each other. Drill at each mark, tap in anchors if you need them, and hang one frame at a time. Hang the frame he picks first, so the wall starts with his choice.
Level and step back
Set the level on the top edge of each frame. Adjust the wire or hook by small amounts. Stop when the frame sits level, not when it looks perfect from six inches away. Then stand back at the doorway and look at the whole wall together. Move one frame if it fights the others. You can always move a hook later.
What to do tonight
Pick the wall, leave the frames in their boxes, and tell him you will start with the one he chooses tomorrow. Ask him one question about her, not about the wall. That question is the real start of the install.