Just gonna brainstorm here:
1. Mercury-Jupiter-Neptune conjunction, he definitely expands the truth and is delusional about it. Although he has a vivid imagination, he probably gets lost in his own mind easily and can't help but lie. It's probably a bit like storytelling to him. Maybe he doesn't even realize he is doing it? He just starts talking and next thing he knows he is exaggerating the truth. Maybe in an attempt to make himself seem more interesting or more confident?
2. Venus conjunct Uranus, he definitely needs independence and freedom in relationships and can be a bit hot and cold. If he feels the least bit restrained he will act out and do something rash to distance himself. He is not good with close relationships.
3. Sun in the 12, he has identity/ego issues. Maybe he is secretive because he is unsure of who he even is or who he wants to project into the world. He seems to lack confidence at his core. He might even overcompensate for this low self esteem by acting out for attention. Like his Sun is crying out for help. It's like he's constantly questioning who he is. Maybe as a child he felt as if he wasn't seen or recognized properly so now he has a difficult time establishing an identity.
4. Saturn conjunct his Midheaven, he is very ambitious and cannot succeed without a lot of hard work. Like he wouldn't have opportunities just fall into his lap. Saturn is also sextile his Sun and rules his ascendant, so its a very important planet. Maybe he is so secretive/lies because he is embarrassed of his shortcomings? I don't know if he is successful in his career but I feel like he is very ambitious and motivated and could be very embarrassed is he doesn't attain success.
5. Mars conjunct Pluto, life or death situations. Extremely competitive. This is the kid who gets pissed off if he doesn't win that game of Monopoly. This is that grown up who will undermine others in order to attain success. He takes what he wants by force and by any means necessary. This can be very manipulative and can make others feel violated.It feels like he has trust issues and wants to establish dominance in his interactions from the get go. With Mars in Libra he wouldn't overtly do it but would rather do it passively. He may also have issues surrounding sex. Like power issues, seeing sex as a violating act rather than a intimate, unifying act between two people. If he could overcome this then he would go the opposite way and see sex as a deeply transformative experience. It's all on a spectrum.
6. Mercury sextile Pluto, manipulative speech. He knows what to say to get to the core of a person. Very intense speech and possibly dark thoughts. He has a natural understanding of human psychology and how to use this to his advantage if he chooses. He seems to have some power issues going on. Maybe he had issues with his dad when he was young? Just a guess
7. What I find interesting is his 11th house stellium and his North Node in the 5th house. This suggests that in a past life he had many friendships/a big network of peers; however, this is uncomfortable for him now. He feels uncomfortable with close relationships and does not like to let people close, which is perhaps why he is so secretive and prone to lying. He lies and is private to keep people away. He has difficulty putting others before himself. He has very little soul experience in regards to being close and intimate with others and incarnated in this life to learn this lesson. In this life he must learn how to establish a close and open relationship with another person. He must also learn to create his own identity, as the 5th house is also the house of creativity and identity. Once he establishes his true identity and gets a real sense of self he will have an easier time letting people in and being honest with them.
I get the feeling he has low self esteem and it scares him because he desperately wants to be powerful, so he lies or manipulates to dominate situations. He has identity issues and issues letting people close. He probably fears that, if he lets someone in, they will see all his shortcomings/see him for who he really is. He needs to develop a strong sense of self and be confident in his identity before he can have a healthy relationship with another person.
That's just what I get from the chart but I could be totally wrong lol I hope my post is somewhat easy to follow! Sorry if it's scattered