First, let's talk about addiction.
You say that you feel he is addicted, largely because he plays 25 hours a week, you "know that this type of video gaming is not healthy", and because it has adversely affected your relationship with him.
Addiction, specifically in this case a behavioral addiction, is the continued repetition of a behavior despite adverse consequences:
The essential feature of behavioral addictions is the failure to resist an impulse, drive, or temptation to perform an act that is harmful to the person or to others (4). Each behavioral addiction is characterized by a recurrent pattern of behavior that has this essential feature within a specific domain. The repetitive engagement in these behaviors ultimately interferes with functioning in other domains.
-- Introduction to Behavioral Addictions
The DSM5 describes "Internet Gaming Disorder" in the section on "Conditions for Further Study". It defines the criteria as:
- preoccupation with such games
- withdrawal symptoms of irritability, anxiety, or sadness
- the development of tolerance
- unsuccessful attempts to control the behavior
- loss of interest in other activities
- continued excessive use despite knowledge of psychosocial problems
- deceiving others regarding the amount of time spent gaming
- use of this behavior to escape or relieve a negative mood
- jeopardizing/losing a significant relationship/job/educational opportunity
Note that the categorization under "Conditions for Further Study" is because not enough information exists to warrant inclusion as a formal disorder.
In all honesty, nothing you've described, with the exception of the deterioration of his relationship with you, sounds like video gaming has done substantial harm to him, or that his interest level has gone beyond healthy.
Looking at the criteria above, are there more signs that you've seen that are causing concern? Has he decided to quit, and failed? Has he agreed with you that his playing is harmful? Does he show signs of mood changes, particularly when other activities prevent him from playing? Does he lie to you and others about how much time he spends playing? Has he lost friends because of gaming?
Despite playing games to a degree you find worrying for more than 5 years, he is doing excellently in college. You say he plays, on average, about 25 hours a week. That's actually not so much when compared to what some people who are playing video games to destructive levels average. Some people play as much as 40 or more hours a week.
It is still a lot, and I can understand your concern, but if he's self-conscious about his looks, introverted, and spending a lot of his time studying, it may not be a warning sign in and by itself. If, has been suggested in comments, a significant portion of his time spent on video games has been involved in working with the design of the game, via mods or changes to the open source code, you may actually want to look at at least some of that time as homework, rather than play, especially given his area of studies.
Keep in mind that there is a lot of socialization that can occur in online games. He may have more friends online than you may be aware of, and while the idea of "online friends" may seem foreign to you, many people today are finding engagement in online communities to be a socially acceptable compliment to other social activities. If he's self-conscious about his medical condition, and generally an introvert, online socialization may be a very appealing alternative.
In all honesty, if you've discussed this with therapists, and your husband, and no one except you feels that this is a problem, you need to honestly ask yourself: is the problem the video game's impact on your son, or is it your belief that video games are "harmful", which, in turn, is causing you to criticize your son's behavior to the point that it is harming your relationship with him?
Addictive behavior is rarely subtle. The fact that your husband does not seem to share your concerns, nor professional therapists, is not pointing to this being a problem on your son's end, especially considering his academic performance.
I suggest the first thing you should do is talk to your son, and ask him why he plays so much. What does he get out of it? Why does he enjoy it? Does he have friends online? Is he playing, designing, or both? If both, how much time on it does he consider " work"?
Open communication will be a great starting point for determining where to proceed next.