This is actually a thing, Taiwanese Mandarin accents are difficult to the point that even Taiwanese people acknowledge it because the other way around with another accent is usually easier to understand
Imagine if the OP sent their son to some other Chinese diaspora and comes back with fluent Wenzhounese and is completely unable to communicate with 99.95% of Mandarin and Cantonese speakers
since southerners including taiwanese don’t distinguish s/sh, z/zh, c/ch, and in some cases r/l and f/h, it could be a bit difficult to listen and understand for beginners.