What can be done about a disruptive classmate? There is a classmate who repeatedly interrupts the flow of lectures in various ways. We assume he has learning difficulties of some kind due to how he acts and because he has an assistant with him in most lectures.

Generally, it will be things such as asking an excessive number of questions. Often, in the middle of going through something, he will interrupt and ask the lecturer to repeat something said or written half a page ago–or will interrupt to try and jump in with the answer to the problem we are working through as a class. Or a question on a semi-unrelated topic, or on a different method of solving the problem will be asked.   

The issue with these questions is they will be blurted out in the middle of an explanation, breaking the flow, and are extremely common, taking valuable time away from the lecture.

Additionally, he will often tell the lecturer to stop or go back a page so he can take a photo of the screen with his phone–and will often take up to 4 minutes before the lecture can go ahead.   

Our lectures are recorded and posted on the student's area of the university website within a day, and previous years lectures are also available–with both the written and spoken information.

Overall, I would say these disruptions take up around 15 minutes of each 50 minute lecture.

It is probably useful to note here that I myself have learning difficulties–which means these are incredibly distracting and disruptive to me personally, more so than my classmates–although in conversations I have gathered that several others are also feeling this is impacting negatively on their education.
You would handle this the same way you would handle anything in class that impacts your ability to learn.  You contact the instructor, and suggest that there is something disruptive going on.