The easiest musical instrument to learn varies from person to person and depends on factors such as musical background, manual dexterity, and musical preferences. However, some instruments that are often considered easier to learn and require less patience include:

Ukulele: Its small size and simple chord structure make it a good instrument for beginners.
Recorder: A wind instrument that is easy to play and relatively inexpensive.
Keyboard/Piano: The basic concepts can be learned quickly, and it is a versatile instrument that can play a wide range of musical styles.
Ultimately, the quickest and easiest instrument to learn is the one that you are most motivated and passionate about.

he can't sing and play this one at the same time though. And that's his goal.

Also, the recorder might be easy to play, considering you just need to blow on one end, but the ****ing thing is a horror to listen to for everybody around. To actually learn how to properly use it takes time (actually I think it takes more time than piano – which you mentioned – as with the piano you always hit a just note. The same wasn't be said for that nightmare of an instrument that is the recorder).

A person learning how to play the recorder might be just as annoying to everybody who has to be in the same apartment as a person learning to play the violin.

I think it's voice. You use it all the time

It comes down to natural ability. We all people who can play any instrument...even with no lessons. I believe that it is generally a genetic thing...something that used to be called "musically inclined" back in the day, involving a good ear and good coordination. Without being "musically inclined", no instrument
will be easy to learn....sorry.

He already sings

Most definitely the piano! It’s easy to understand, just makes sense. Still agree that it will depend on your motivation and how naturally gifted you are musicwise. One of my friends plays a lot of different instruments, but he’s just plain talent. Learnt them all in no time.

The most exact answer to your question would probably be the triangle, but I doubt you're looking to improve your triangle skills

If you want to understand them all to a degree, I would recommend the piano - it does require patience, but it's a great teacher to learn other instruments as well.

Rhythm guitar would be the most beginner friendly, I guess - but you're not going to learn much theory this way, and guitar skills are harder to translate to another instrument.

In my view, the ukulele is one of the instruments that require the least patience to learn. Its compact size and simple chord structures make it incredibly beginner-friendly. You can strum along to your favorite songs with minimal effort, and it's a fantastic gateway into the world of music. If you're keen to explore the ukulele, https://www.ukulele-tabs.com offers a wealth of chords and tabs for various songs. The ukulele's cheerful tones and ease of play make it an excellent choice for those looking to quickly enjoy the pleasures of making music.

I tried learning piano and it turned out to be more difficult than playing a guitar.

Singing: Since you already enjoy singing and find it natural, continuing to develop your vocal skills could be a rewarding path. You can explore different vocal techniques and styles, and perhaps even take some lessons to further enhance your abilities.

Percussion: Instruments like the tambourine, shaker, or hand drum require minimal movement and can be a fun way to explore rhythm without the complexity of melodic instruments.

Keyboard: While the accordion may have been challenging, a keyboard or piano can offer a different experience. The keys are laid out in a linear fashion, which might be easier to grasp than the accordion's button layout. You can start with simple melodies and chords and gradually build up your skills.

Digital Music Production: Since you've already worked with FL Studio, you might enjoy diving deeper into digital music production. You can create complex compositions using virtual instruments and software without the need for physical dexterity.

Ultimately, the best instrument for you will depend on your personal interests and preferences. It's okay to try out different options to see what feels right for you. The most important thing is to have fun and enjoy the process of making music!