I think it’s perfectly reasonable to ask whether a student’s poor performance is due to a lack of ability or lack of access to appropriate instruction, a lack of a firm foundation or inefficient study habits. We asked this for our kids when the issue came up, were pointed towards “other kids” who were “doing better”, and advised to stop then and there, and found that while we could not guarantee degree of improvement, it was possible to “clean up” after poor instruction by getting a good diagnostic and working from there.
And, then, lo and behold, sometimes our kids were the ones who were “doing better” even after tutoring stopped, and other parents (including a couple teachers) took us aside to ask what had worked. Sadly, in a k-12 setting, few educators showed any curiosity about the source of the improvement, so were likely no help at all to the next student who came along with similar issues.
Other times, we saw no improvement and pulled the plug in spite of protests from providers that we had not tried long or hard enough. It is helpful to get a good idea of what needs to happen, and then how to tell whether it has actually happened or not, ideally when a third party who is not aware of the tutoring or therapy notices a change after a couple of months or so.
Right now, OP’s SS has an interest in becoming an engineer and perhaps this interest will motivate him to excel at math,even if he did not do so previously. It is truly a gift to him to give him the chance to find out whether this can be realistic. One suggestion I would make would be to take a look at the Coursera class, “Learning How to Learn.” The professor teaching this class, Barbara Oakley, began STEM studies at age 26, after earning another degree and after a history of poor performance in high school STEM classes.
I do not think that the OP is unreasonable for asking whether he will experience sharp improvements. Every parent asks this when considering music lessons, athletic coaching, all manner of tutoring and remedial instruction, therapies, unless perhaps they have unlimited funds at their disposal. And even with unlimited funds, there is the manner of wasting time that could be spent on other pursuits.