al-hamdella i am currently studying Mathematics; inshallah in less than a month i will be entering the second semester of my junior year so i have a pretty good idea of what it takes to be a math major.
So basically the first two years of your study- for me at least- is quite simple. Because you can't get into real math in most universities until after you finish the core requisites which are Calculus 1,2, and 3. This could take 1.5 or 2 years depending on whether you take calculus 1 your freshman year or an introduction to calculus class.
During this time it is almost impossible to take any other math class, so your schedule is basically full of electives and english. Once you've completed those 3 class you are allowed to take high level math classes at will. The nice thing is that after the 1.5 or 2 years of those core classes you still have the opportunity to potentially change majors to almost any engineering or art and science major without losing a lot, because all of those majors also usually require calculus 1-3 and the same electives and english.
After you finish calc 1-3 you can pretty much say that 3/4ths of your remaining classes you will never see a number in. The classes that will have numbers will be classes such as stats, linear algebra, and differential equations, but other than that the rest of your classes will be proof based.
Your university will most likely make you prove everything that you ever learned from why does 1 + -1 = 0, why do two negative numbers multiply to be positive (surprisingly hard to prove), along with algebra concepts. More so, every formula you are taught in calc 1-3 you will revisit in order to show how the person came up with it, and be able to emulate of proof of the theorem.
Classes that are proof based are very numerous, but they include classes such as: real analysis, fundamentals of calculus, abstract algebra, complex analysis, linear algebra 2 and advanced differential equations. The last two prove all the formulas and theorems done in their pre-requisite class.
At the current moment: proving such things seem daunting, and potentially boring and a pain. Well yes it is a pain, a huge one; not going to lie. And they'll be many a day where you miss the days of introduction classes where you could raise your hand, mention a few of the vocab words of the chapter and the teacher would nod her head and tell you nice point without asking you anything. In a math class you must be exact and precise, you cannot make a claim without the appropriate proof to back it up.
Because most of math major doesnt have numbers being able to do mental math is pointless. The classes that do possess numbers are usually only in the first 2 years and are a focus on integration and derivation (basically the manipulation of formulas to produce alternate formulas that tell us the characteristics of the original formula) this doesnt require you to need to know mental math.
not to make the post too long, you can PM me if you have any other questions or ill check back here soon, but a math major can either be the major of the most work out of all the majors in the world or the least work ever. No matter how much Allah (saw) has blessed you with intelligence, no one is born knowing the different parts of a cell or the names of all the bones in the body, so majors such as biology have students who always have to study, though some people have better memories than others.
Math on the other hand has no memorization, its pure understanding. This could happen for someone during class as they listen to the professor then not have to open the book at all for a test or quiz, or they could spend hours looking at books and notes and not understand the concept. Its a lot of "clicking" moments, and unlike other majors where you could get a D in bio 1, but be amazing at chemistry or zoology- most of math is connected in a way that if you didnt understand it in Calc 1 or some class and you breezed by without it- that sooner or later its going to come back in a more difficult form where you not only have to be able to solve it, you'll have to prove why the method is correct.
You dont have to love math to study math in college, you'll learn to love it- you just have to be curious enough to want to understand everything
instead of just learning a formula and how to apply it- to know where the formula came from).
idk anything about an Open University but in Maths, whatever you put in, is what you get out. You have to do LOTS of question just to get the hang of one rule! But if he really tries he can do it Inshallah 
