A spare key is the easiest thing to ask for and the easiest thing to get wrong. Most fights over keys are about permission. This piece is about fit: whether the key you have will turn the lock he actually uses, and who holds the copy once it does.
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Find the lock he uses, not the one you remember
Walk the house the way he walks it. Note every exterior door, the garage side door, and any storm door. A house often has a deadbolt on the front, a knob lock on the back, and a thumb turn on the inside that makes the outside key useless when it is set. Write each one down with its location and which way the bolt throws. Do not assume the front lock matches the back. A second key that fits the wrong cylinder is worth nothing on a bad night.
Read the cylinder before you copy the key
Look for a manufacturer stamp or code on the cylinder face or the original key. Compare the key's grooves and depth against the copy you plan to order. Check the keyway shape. A key that looks similar may not turn the cylinder, and a worn copy can jam halfway through the turn. Bring the original key and a clear photo of the lock to a locksmith. Ask whether the copy is cut from the original or from a code, and whether the cylinder is the one the door was built with. Ask whether a replacement cylinder would change the key, and get any quote in writing before anyone takes the lock apart.
Check the door, not only the key
A correct key can still fail if the latch and strike do not line up. Open and close the door with the key and without it. If the bolt scrapes, the door sags, or the key only turns when the door is pushed, the problem is the frame or hinge, not the copy. Write down exactly what you saw and when it happens. That note is the first thing a locksmith or door repair person will ask for.
Decide who holds the copy before it is cut
Every key you make is a key someone else can use. Ask him in writing, even in a short text, who gets a copy, where it is kept, and what it is for. A plain starting line is that the key is for getting in when he cannot get to the door. Keep your copy somewhere he knows about. If he refuses a key, record which door you cannot open and what that gap means for his safety, then take the question to people who can advise on it.
Keep the legal line in view
Do not let a key become a way around his refusal. Entering his home without permission, or changing the lock while he is still able to decide for himself, can create serious problems for you even when your intent is kind. This is general information, not legal advice. Before you go beyond a copy he has agreed to, talk to an attorney, and use the Local Help in St. Louis page to find who to call.
When the key fails in an emergency
If he is locked in or the door will not open and you believe he needs help now, call 911 and tell the dispatcher which door is blocked and why. Do not wait for a spare key to arrive or a locksmith to be found. The key is for ordinary visits, not for emergencies.
The next move tonight
Tonight, list every exterior door with its lock type and the key you have for it. Pull the original key from the door and read the stamp or code. Then call one locksmith, send the photos, and ask whether a copy will match your cylinder. Write the answer down. Send Dad a short message naming the door, the key, and who keeps the copy. If he says no, start with the Start Here page and work from there.