A couple of people have suggested that he might be okay if he has some “natural ability” or “natural talent” in mathematics. I’m not completely on board with that assessment. There’s a lot of educational research out there showing that students are far more successful if they think of skills (particularly in math) as a result of hard work and effort instead of innate ability. They also do better if they are praised and encouraged by the adults in their life in terms of their hard work and effort (“You worked really hard on that, look how your effort paid off!”) as opposed to their perceived innate talents. Students who encouraged, unsurprisingly, do a lot better than students who aren’t - no matter their original test scores.
There’s also quite a bit of research that supports the idea that “natural ability” in subjects (math particularly) is a subjective perception, and particularly at his age may have more to do with how much effort he’s put into math so far AND how much exposure he’s had to mathematical subjects. So rather than trying to figure out whether he’s got some kind of “natural ability,” I’d focus on his interests (is he truly interested in math and engineering now, or is it sort of the career-of-the-week? I was frequently “suddenly” gripped with the desire to be a Something in high school, only for it to be Something Else a few weeks later) and his efforts (is he a hard-working student who is willing to put in the time to get better in an area?)
Also, be wary of pushing too hard. He doesn’t need to spend his summers trying to jump ahead a year in math unless that’s something he actually wants to do.