MountainSide, he may not need a MIDI interface. Using a 5 pin to USB MIDI adapter he would connect the MIDI part of the equation directly into his computer.

Tangmo, the https://www.ebay.com/p/Batige-Dual-2-RCA-Male-Plug-to-XLR-Adapter-Spiltter-Patch-Cable-Cord-Audio/21004206847?iid=253849516742&chn=ps

will merge both left and right channels into a mono signal. If mono is ok this then this leaves your other input for you guitar or vocal. (note - if your synth has a mono out then just plug it into your interface - no need to buy anything) If you want stereo then you will need an adapter like this:

https://www.ebay.com/itm/XLR-M-Male-2-to-RCA-M-Male-2-Stereo-3ft-Adapter-Converter-Changer-Plug-Cable/152965606238?hash=item239d75eb5e:g:3n0AAOxyQ45Q~5Jc:sc:USPSFirstClass!14420!US!-1

This will use both of your inputs.

BUT if you use a DAW you can record your stereo synth on one track, unplug your synth and plug in your guitar and/or mic and record on another track. (note that you can also record as many tracks as you want using your synth, guitar or mic) It's only if you want to record everything simultaneously that you will need to buy a different interface.

When you are at the checkout line and they ask if you found everything say "Why, are you hiding stuff?"

64 bit Win 10 Pro, the latest BiaB/RB, Roland Octa-Capture audio interface, a ton of software/hardware