I agree, get him diagnosed (your general pedi can help you, and the school can do the psych testing for free, although they’re less likely to diagnose him than a private psych would), and get the accommodations in place. Things like permission to turn in work late (and you’re gonna have to help him keep organized on it), teachers giving him their notes for class lectures if he has trouble focusing long enough to take notes for 45 minutes straight, extra time on tests, selective seating in the front row, they all can help.
It sounds to me as if he’s got a wonderful early intense interest in an unusual field for mid teens - so encourage it! Have him take the free online courses in economics on the website modernstates, and the CLEP exams for them too (it’s all free). And if he exhausts that, then have him continue economics courses at your local college - many of them will allow high school students to take one course a semester for free. You think that the prodigy math geniuses at top IVY schools all were straight A students in English and History, too? It sounds as if your young man could be the kid who applies to top schools for economic theory, having already taken eight or more college level econ courses before even applying. They don’t get many like that. If his SAT scores are high, and his grades aren’t terrible, they’ll take him.
Be happy. He may have found his calling, and if this interest passes, I’m sure another will quickly take its place. Get him the support he needs, encourage him, and know that executive functioning in these kids improves with support structures (habits and routines and checklists) and with age, as the brain continues to mature into the early twenties.