I had a cath and a stent too. Your boy is 17. I was 81 so we are both on the exremes. The cardio had 20 years experience and I calculate he had done at least 4000 caths. Don't fret. They know what they're doing
I am puzzled why the doctor did not explain to you the reason for a heart catheterization. Of course it must be related to why your son needed to go to a cardiologist in the first place -- symptoms? Prior diagnoses?
Generally speaking, a heart catheterization is performed to assess coronary arteries (#, locations, any narrowing), heart valve (leakage or narrowing) or heart muscle (thickness, movement, or any holes where there shouldn’t be one). It can also record pressures inside each heart chamber reached by the catheter.
Catheterizations are rarely performed in most 17 year-olds. Your son must have a congenital abnormality which the doctors have discovered either by echocardiography or his syptoms. I will also add that it is a very safe procedure with low likelihood (but not zero) of adverse results/harm.
With limited information on your son’s health history, I can only comment this much. Hope this helps you.