Judo has two basic principles
Both principles are what makes Judo "gentle".
The first one is the technical principle: seiryoku zen'yō (精力善用, maximum efficiency, minimum effort).
This one makes Judo "gentle" in the sense that you do not waste any strength and effort with overly aggressive and brutal moves that do not contribute to your main goal: Defeating the opponent. Thus, every ounce of strength you use unnecessarily violates this principle.
The second one is often called moral principle: jita kyōei (自他共栄, mutual welfare and benefit).
This one makes Judo "gentle" in the sense that not only in your martial arts practice, but also your everyday life, you are supposed to focus your actions on joint goals and consider the needs and interests of others. This also applies to randori and technical training and means that you should always support and help your fellow students and move and fight in a way so that both can learn as much as possible.
Physically strong beginners
Let us be honest: What you describe does violate both principles profoundly. This actually is a very common occurrence, not only in Judo, but also in BJJ. It often takes a long time for older beginners to reach the point where they relax enough to be able to actually execute techniques correctly. For good technique, you need a mix between relaxation and explosiveness, while the former enables you to fast and fluid movements in the first place. Tension is your enemy.
There are two things to be said:
First: Using strength hinders not only your advancement but also his own technical development.
In the documentary 'Be strong, Be Gentle, Be Beautiful', Keiko Fukuda - first woman ever to be awarded the 10th dan - theorised about women often being better Judoka technically speaking. Not because of their gender as such, but because they tend to simply not have the strength to power against a partner and therefore have to learn efficiency early in their career.
And she has got a point there: The smaller and weaker beginners comparatively are, the faster they tend to advance technically.
Second: This type of student repels other students.
I've seen it a thousand times: Newbies come, have fun, and are pounded by an over-motivated training partner. That hurts group dynamics and makes you lose (potential) members. I've been in your shoes as well. It took me like three years to finally be at a technical level that allowed me to beat my former nemesis (a level he never achieved). But had it not been for the rest of the group, I would not have continued to go there. In the end, nobody will want to train with him, even less like him. That cannot be in his interest either.
What you can do
I think there are four main strategies that can be applied subsequently
- Speak to him. I gave you some solid arguments here that many reasonable people can accept. It may also help to point out the moment he uses strength like in "Hey, you are tensed all over again, how am I supposed to learn anything that way?" If he is reasonable, he will try to work on that. If not, proceed to
- Speak to your instructor. He is aware of Judo being supposed to be gentle? Well, all these arguments should appeal to him as well. It is in his best interest that the spirit of the art and the interests of his students are realised by his example and actions. Nevertheless, it may be that he does not care enough to actually do anything about it or his actions are fruitless. If so, proceed to
- Avoid any training with this guy. It is true that you can learn most from advanced partners anyway (rightfully pointed out in the answer of @mattm). And it is even positive for them as well, as you sometimes are really baffled by how beginners are moving and reacting if you are advanced and only train with advanced students. Also, trying to help you improves their understanding of the technique (remember: mutual benefit!). But rejecting to train with another student is rude and against the etiquette. Therefore, it may happen that this guy (and/or others) still overshadows your experience. If so, try
- Sometimes it is part of the culture of a club or training group that they are brutal and stretch "gentleness" to the point where it simply is not gentle and in accord with the above-mentioned principles anymore. Sometimes they do not care if members do that. If the training with this group remains unsatisfactory, sometimes the best solution is to go to another club and try them.