Yeah I 100% feel the same! He read an entire book on "Mathematics for Machine Learning" which he found pretty easy. He skimmed over the "now let's apply this in code" sections but the maths behind it all was fine.
The pros of starting in web dev is he should be able to get an entry level role pretty quickly, it's probably the biggest market and he does find it interesting. My belief is that as long as you keep digging you'll find a rabbit hole in any area of programming and his skills will be valuable there. e.g. in web dev you can move into cryptography, WASM, encodings, color spaces, parsers, compilers etc etc. Also, while it's good to have an understanding of the maths when doing data science stuff you're not that likely to be actually using or innovating the maths and most of your time will be spent cleaning data and importing libraries? Also he's more of a pure maths abstract algebra guy and less interested in stats.
The cons of starting with web dev is that you're way less likely to need his maths skills.