If the DVD recorder doesn't have a tuner then why does it have a RF input and output? What model is this?

What you need to do is the following:

Connect the aerial to the STB RF in using coax.
Connect the STB RF out to the DVD recorder RF in using coax.
Connect the DVD Recorder RF out to the TV RF in using coax.
Connect the STB VCR SCART to the DVD Recorder EXT SCART (AV2) using a SCART cable.

Here is your choice now. Do you want the STB output to be via SCART or the DVD Recorder output to be via SCART? It'll be a lot better picture quality than RF. If he watches more stuff straight off the STB then use that otherwise plug the DVD Recorder in.

Connect the DVD Recorder TV SCART (AV1) to the TV SCART using a SCART cable.

or

Connect the STB TV SCART to the TV SCART using a SCART cable.

To set it all up, just set the DVD recorder to accept input by the SCART if you want whatever is off the STB and via the RF input if you want whatever is off the normal terrestrial analogue broadcasts. Depending on your choice above, set the TV to tune to a RF channel for one device and the SCART input for the other. You can always set it to tune to both RF channels anyway so it's easier to swap the cables over sometime later.

You don't need the SCART splitter you've got as that is for what you have one output device and multiple input devices.