When you compose this message to your father, and send it, it should be sent as a text message. Have you tried to send it, or are you discouraged by seeing "iMessage" while you're composing it?
Even iPhones may be out of WiFi (or cellular coverage allowed by iMessage) range when you send them what would otherwise be an iMessage.
If the message is blue when sent, it is an iMessage. If green or white, it is not.
Ok, lemme get this clear so that I can rectify this as fast as possible. I am a developer and I am very, very, very knowledgeable of iOS, and Mac os, unix, windows, etc..... I have a computer science degree also. Please don't take this personal anyone, but the simple things like what was listed above I am very aware of. PLEASE, I really need help, and I am positive I am not the only person that has experienced this. I am looking for a solution, not an explanation of how the system works. I really want help, I'm not being cocky, but I need real answers. Thanks in advance. I sent a reply to a txt my father sent me, and when he called me he asked me why didn't I respond to h is txt. Lol, I told him I did, and went looked and realized that it was a iMessage sent. I called him, and asked him to turn off iMessage on his old iPhone and turn off the wifi, the sim is deactivated. No message was sent to any device. On my phone it confirmed that by not giving the delivery confirmation under my txt to him. When I turn off iMessage globally I can txt him. But I don't want to do that. It shouldn't be like that.
Your father should have turned off iMessage BEFORE getting rid of the 4S. He can fix this by putting his sim in any iPhone & turning iMessage off, or by logging into his Apple support profile & removing the 4S...assuming he setup a support profile with Apple. If not, he'll have to do the sim in another iPhone deal to fix.
I had the same exact problem as your father and as another post mentioned, I also used my new sim to disable iMessage with success. I love apple products and praised my iPhone but I was thoroughly disappointed by the new iPhone so I purchased a samsung Galaxy S3. I love the phone and was enjoying all of its features, except for the text issue exactly as you described. Nobody at AT&T told me that this would be an issue so I didn't know to deactive iMessage before setting up the new phone.
Long story short, I turned off the samsung, put my sim into my old iPhone and turned the iPhone on. I was hammered by tons of iMessages that were queued while the phone was off. I Immediately went into settings >> SMS >> disable iMessage toggle. I turned everything off and put the sim back into my droid and turned it back on. People that could not text me before were suddenly having their messages come through to my droid now. I also tried to go into my Apple support profile afterwards just to see what it was all about but my iPhone 4 was not listed so I'm not sure if there's any correlation to the support profile and the iMessage. I don't know for for sure but I suspect that it may just be like an old wives tale. Anyway, the problem was resolved by simply putting in the old sim and turning off iMessage, then turning off the phone and swapping the sim back. If you're on verizon by chance, I'm not sure. Good luck.