Was this character able to become one with the Force at the end of The Rise of Skywalker?

Question

After the final battle in The Rise of Skywalker,

Ben Solo uses the Force to give his life energy to heal and revive Rey. The act of sacrifice takes a toll on his own life and he dies.

Now, if I remember correctly his body also vanishes. This would seems to suggest that he "became one with the Force" and should be able to project his image as a Force ghost. However, at the end of the film

both Luke and Leia appear to Rey, but Ben (to me, notably) does not.

Can it be confirmed that he was able to become one with the Force, and if so why did he not appear to Rey alongside Luke and Leia? This would have been a nice homage to the three appearing to Luke at the end of RotJ, Obi-Wan, Yoda, and Anakin.

Answer 1

Yes, he was a Force Ghost. His body disappeared, in current canon this makes him a Force Ghost.

In an interview, Star Wars screenwriter Chris Terrio explained the rationale behind him not appearing alongside Luke and Leia, also insinuating that Ben Solo was a Force Ghost by accepting that he is an option to appear, so that supports what we already know.

What was the rationale behind only showing Luke and Leia’s Force Ghosts on Tatooine and not including the remaining Skywalkers, Anakin and Ben?

We absolutely discussed who would be there at the end. It’s not as though those Force ghosts will never appear to Rey now that she really is the first of the new Jedi. I think she has all of those Jedi behind her. J.J. was pretty clear about the idea that he didn’t want to take away from the moment of Leia finally appearing as a Force ghost and the twins finally being together.

[...]

It’s a fair question from fans because it’s a question that we debated endlessly — about what the final shot of Force ghosts would be. We spent hours and hours talking about this and debating it, and we decided that the moment when the Jedi have to be there for Rey, when it dramatically counts, is when she hears their voices. So, seeing them all at the end would be a lovely grace note, but we thought that Rey seeing her two masters, two Skywalkers, was stronger.
_{Hollywood Reporter}

The final sentence here being the short answer to your question:

we thought that Rey seeing her two masters, two Skywalkers, was stronger.

Answer 2

It would appear that the Living Force decided that he was worthy (based on his sacrifice) and accepted him.

He had given Rey back to the galaxy. It wouldn’t atone for the darkness he’d wrought, but it was what he could do.

Ben Solo had no regrets as he collapsed to the ground. The Force reached for him in welcome. His final awareness was of Rey, clasping his hand with her own.

_{^{Star Wars: The Rise of Skywalker: Expanded Edition}}