Depression isn't simply solved by saying "just go do something that makes you happy". When you're depressed, if you are truly feeling miserable, it is very hard to push yourself to do something, anything. Finding a meaningful hobby is very much harder than you think if there seems to be so little that brings you joy.
Can you see the downwards spiral here?
This makes it hard to find joy in doing things
This makes it hard to move yourself to do those things, to do anything
Do nothing instead, which is something you feel guilty about, making you even more miserable.
It is really really really hard to break this spiral. Especially when looking at it from the outside, because it seems so simple, just go do something, right?
Instead, this spiral makes it easy to slip into a habit that makes you feel slightly better, even if only for a while. This can easily lead to an addictive habit like gaming and/or substance abuse.
So how could you help?
This is really hard. I don't have the answer. Honestly, if the answer existed, depression wouldn't be that big of a problem. Instead, it is important to get help from someone who is trained to deal with these situations.
This only really works if your friend cooperates, so they have to see the need (and commit the time) for therapy, else it is just a waste of time and money. This means you need to convince your friend that:
They have a problem. This is hard enough on its own. The important part when trying to convince them of this is to be careful that you are not blaming them. They can't help their depression, they are a victim.
That this problem is substantially big, that they can't solve it on their own, that you can't solve it for them either, that there is help available, and most important of all that it is not a bad thing to ask for help!
And honestly, if he thinks he doesn't have time for it during his studies, if there is no immediate risk (selfharm for example), it is not that bad to wait a while.
So is there nothing that you can do?
Why don't you invite him along on your activities every now and then. While it might be hard for him to get moving on his own, he might get some motivation or help from just doing things together.
But be sure to take the initiative, don't sit back thinking "if he wants my help, he can ask for it". This is one of the big reasons so many people are depressed without telling anyone about it. "If I feel terrible about myself, I sure as hell shouldn't bother others with my problems."
In your question you edited:
AS tinkingerbell said it seems like i want to force a lifestyle on him he does not want. THe problem is that he DOES ENJOY stuff like this, again the example of going bouldering with me. We did that a couple of times, and had a blast. It was really fun for both of us. THe problem is he has no self drive and would never go alone with his initiative, only if someone drags him to do stuff. Then he really anjoys it again. I actually was the same at his age. At that time it was my brother that dragged me everytime because i always enjoyed it. Now i have drive on my own. He sadly just doesnt have that.
This is good, he enjoys something, but clearly he lacks the motivation to do it himself. This is something I recognize from myself. For a while I just went jogging a few times a week, to wake up before my driving lessons. But once those were finished, the big stick that kept me jogging disappeared, and it again becomes more and more easy to fall back to those old habits of gaming etc.
The place where you could break the cycle in the top of the answer would be point 3. They have trouble moving themselves to do things, so they need you to drag them there (not literally). Be that stick, give them those few hours that he enjoys, and make sure he keeps doing it. Don't rely on him to take the initiative, but be proactive in your help.