I’m not sure you are going to be successful with the ‘standard’ RF solutions as I don’t think they are spatially aware enough. Example: I believe you don’t want it to trigger when he is inside house.
There a spatially aware RF devices, but I have not found any that I could buy and hack for reasonable price. The ‘gold standard’ devices are the RF keyfobs for cars. The keyfob and the receiver know whether the keyfob is inside or outside the car so you will not lock the keyfob in the car…
I’m still saying look at NFC, ‘low tech’, inexpensive, battery free, relative low worry if he looses it.
Put the NFC reader in the window (have had good luck with NFC reading thru windows and 1/2 inch of solid wood, but doors a bit too thick) .Put a count down timeout on it after he swipes it. If you have a modern iPhone, they have NFC readers built in and apps to read tags, you can buy a cheap NFC tag and test before you build. Home Assistant has some nice NFC support built in, read some of the blog posts about use cases.