Help! New mono cartridge for Dad's present-need the least amount of tweaking

Why bother with a mono cartridge? A stereo cartridge will work fine!

Keep the 103R! No need for a separate mono cartridge. 

Perhaps the box set can be returned for the stereo version.

I own thousands of mono records and a dedicated mono table.  Mono definitely sounds better with a true mono cartridge.  

Donvito and roberjerman are correct if you don’t want to go big into mono.  The monos will play with a stereo cartridge. Just not as well.

There are a lot of ways to skin a cat.  To answer your question, yes you can buy him a DL102 and it will be very similar in weight and overall dimensions to his 103, therefore, it will be easy enough to switch back and forth.  All else being equal it should sound better too, because as a mono cartridge it is only picking up the signal in one plane off the record.  That means in theory it could/should be quieter.  In reality the difference is often negligible or very slight.  That is another way of agreeing with the guys who are saying why bother.  Another option would be to get him a summing switch which would sum the left and right channels into one mono channel electronically.  This does exactly the same thing as switching cartridges and is for preamps that don't have a mono button.  You can get such an item for short money from KAB Acoustics and it would be my recommendation.  Finally, if you are a real nutter like me you need two turntables....

 While I do agree that you can have a very satisfactory listening experience with mono LPs, using a stereo cartridge, I have yet to discover a mono LP that doesn’t sound even better when reproduced using either a mono cartridge or a preamp with a mono switch. So you would be doing your dad a favor to provide him either option. But if his only mono LPs are those Beatles albums, maybe it’s not worth the effort.

Audio Technica AT MONO 3You can buy these online for about $120.  Get an Audio Technica headshell with fixed holes and you can set it up in about 5 minutes.  I had one and it sounded great. It did make a audible difference in my system compared to just using a mono switch.