He may be curious as to whether you like him romantically.
He may not quite know.
When he asked you about what you like about him, he may have been trying to get some clues as to what you think of him.
Again, he may not really have any idea as to whether you like him romantically or whether you simply enjoy his company.
He may have reached a point where he knows that he is becoming more and more fond of you.
However, he may be worried that if he were to allow his interaction with you to continue without knowing for sure whether you like him romantically, he may end up falling for you without any assurances that you will do the same.
In asking you about what you like about him, he may have reached the point where he just wants to get a real sense of what your feelings are for him or how you look at him.
If you were to respond with words that make him feel as though you like him romantically, he may feel encouraged to continue on in his interaction with you in the hopes that at some point, the both of you will go out on a date.
If you were to respond with words that make him feel as though you merely look at him as a platonic friend and nothing more, he may take that information as his cue that he may have to slow down in how he goes about interacting with you and perhaps make these interactions a lot less playful.
In other words, he may have to start treating you as he would a platonic friend as well. This way, he keeps himself from ultimately falling for someone who doesn’t feel the same way about him.
A guy could also ask you about what you like about him because he may be trying to get a boost to his confidence.
Being that he already has a friendly relationship with you, he may want to use you to help him feel a boost to his sense of self-worth.
Hence, this is a question that he may not only have asked you.
He may ask this question to other friends that he has.
Every now and then, he may just need a little encouragement or boost to his ego or confidence just so that he feels better about himself.
He may come to these moments when there is something in his life that didn’t work out the way that he wanted and his confidence has taken a hit because of it or has been negatively affected by it.
It may be a situation where he didn’t get the promotion that he wanted at work or he didn’t get the grade that he wanted at a class, or other scenarios where he may have failed at something.
By asking you about what you like about him, he is hoping to hear the kind of words that let him know that he still has value, thereby giving him hope that his future endeavors will succeed.
Hence, he just wants to feel better about himself so as not to focus on the failure that he may have experienced prior.