His mercury is square Jupiter/ascendent.
From astroTarot:
Typical features that this square brings: you have a clever mind with a broad philosophical outlook on life. You love to express your ideas, sharing them with other people pretty readily. Jumping to conclusions can be a problem. You often overlook details. Also, your thinking and studying habits are not always disciplined. So, this can result in sloppiness. With this Mercury – Jupiter aspects, you might vacillate between logical thinking and an optimistic conclusion-drawing bent of mind. So, it can be challenging for you to truly get to the heart of a subject. Your intentions are generally good. But you might promise more than you can deliver. Be careful not to cultivate a reputation for being unreliable, simply because you desire to make others happy with what you say. You can be extraordinarily funny and interesting. You possess an acute but disorderly mind. So, you must learn to be more disciplined, to concentrate more on details. Your optimism can sometimes distance you from reality. You learn mostly through interacting with others and through personal experience. Also, you may travel widely, talk a lot, or be very productive, in whatever you manifest from your mind. Your full potential can be realized by consciously working on your concentration level.
Positive traits: You have the potential to grasp abstract ideas. And you can acquire a good understanding of any subject you wish to study. You certainly have the capabilities and are willing to keep learning and improve your skills.
Negative traits: Concentration and self-discipline may hold you back from reaching your full intellectual potential. Sloppiness may apply to your thinking, in your writing style or how you organize other aspects of your life.
Basically, he could exaggerate things, become over enthusiastic.
His sun opposes Saturn:
The opposition reflects an inner conflict externalized into the outer environment of relationships. The issue of self-esteem and worth is highlighted, and you may experience a degree of hidden insecurity and fears of full involvement in life.
From astromatrix, here is a link to the page:
https://astromatrix.org/Horoscopes/Planet-Aspects/Sun-Opposition-Saturn
The midpoint of Neptune/pluto is at Saturn, and opposing sun.
Here is what John Sandbach has to say about this configuration:
To tap into or be affected by extreme subtleties. The process of refining and transforming energy. Tremendous often overpowering imaginings. Obsessive imaginings and visions. Hallucinations. Profoundly deep idealism. Supernatural occurrences. To be transformed and evolved by one's visions.
And the link to the page, which is very detailed and informative:
http://john-sandbach.blogspot.com/2010/12/planets-activating-neptunepluto.html
But to have a proper evaluation you also have to look at sun opposite pluto, who is you.
From astromatrix:
This suggests that most of your problems and challenges will arise in the area of interpersonal relationships, often created or stimulated by your own unconscious expression of this energy, which requires a deeper understanding and inner adjustment to be achieved before you can consistently benefit from its power.
and a link to the page:
https://astromatrix.org/Horoscopes/Planet-Aspects/Sun-Opposition-Pluto
Your Venus of love and affection is exactly trine Uranus/neptune and square Saturn.
You might like to check that out as well.
Keep in mind, you can exercise as much control and transformation energy over yourself as you fell you need, but you have to be careful about applying it to other persons or to the situations around you.
Sometimes you just have to fly by faith and take it the way it is.
Pluto energy works both ways, towards you and from you.
At the end of the day, you will have to decide how much you are willing to live with this in a relationship, how far are you willing to go.
This is an issue for you to resolve that goes beyond just this person and this relationship.