How to help a junior developer to read other people’s code

In one of my teams I have a developer who struggles to read code that he didn't write. When he needs to read code, the only way he has to understand it is going through it line by line with a debugger. He doesn't seem to be able to get any insight from reading code. He cannot navigate code well. He cannot seem to build a high level description of the code. He is also generally somewhat helpless, but this is a particular weak point. He is by far our weakest developer due at least in part to this issue.

On the other hand, he understand his own code well. My impression is that for his own code, he also has to use the debugger a lot when he makes changes. But, being familiar with it, he can set the breakpoints at the right places and do it fairly efficiently (but still slow compared to other developers).

Are there any resources that can help to develop this skill? This developer has been with us for 2 years now, so my initial hope that being exposed to lots of code will improve this issue hasn't really panned out. I don't have hope that this issue will improve on its own. I am having trouble helping him, maybe because I naturally read code fairly easily.

His position is actually as a machine learning engineer, and he is good at all the hardcore machine learning stuff (ML architectures, overfitting, etc.), but we are developing full machine learning pipelines, so there is a lot of coding as well.