Regarding taxes, what matters most is how much he earns (i.e. salary and wages) each year. Given the NW you quoted I assume he has a high(ish) salary, which makes it all the more advantageous to use tax-advantaged buckets first (IRAs, 401(k), HSA, etc).
As for his total NW, I’m guessing it’s higher than you mentioned if he has $240k in 5 stocks and about $30k left on the mortgage. Traditionally you also include the equity in your home, and you would definitely include e-funds and cash on hand. Not that it matters a great deal here but knowing this might shed more light onto his approach. Already there are some red flags about his approaches (owning just five stocks across two industries).
In terms of getting him to contribute more towards his tax-advantaged accounts, what works for most people is to show them how much more money they could have utilizing them than not. Mad Fientist has some great graphics showing how much extra money one can have by contributing to these accounts (in most cases it’s well north of $100k).
For others, it’s enough to simply show them how much less they could pay in taxes THIS YEAR and ignore the whole tax-free growth argument entirely. IT can be as simply as “hey, for every $1,000 you put into your tIRA you will get $240 back when you file your taxes. That’s $1,440 if you contribute “just” $6k this year”
G’luck.