Firstly, my sympathies. A good friend of mind died far too young working in the US and his parents, also my friends, were understandably devastated. My comments here reflect their experiences and not mine. I'm not a lawyer and there may be others with experience in this field who know better. That said:
What you are being told is probably correct, and the problem is that your son is considered a US person from the perspective of a US entity. Moreover, probate in the US is an expensive and convoluted process, as you might expect from a nation that has 70% of the world's lawyers.
And it may be worse than that, because the US has a nasty habit of taxing anyone whom it considers to be a US person. If that is the case then there may be US tax returns and liabilities to be considered, depending on what basis your son was in the US.
What I can do is estimate for you the value of the 401K account, and then you can decide whether it is even worth pursuing, or better just to write it off. The current annual 401K contribution limit is $18,000 but how much he actually contributed in 8 years would depend on what period this was for - the limit has gradually increased over the years. It also depends on whether he contributed the maximum (most people who can afford to will do this because of the tax deduction). It will also depend on the corporate "matching contributions" which, typically, might be on a dollar-for-dollar basis up to a limit, maybe 3% or 4% of salary.
This link shows the contribution limits by year:
https://dqydj.com/the-complete-history- ... ion-limit/Assuming his stay was recent and he contributed the maximum, then the contribution value might be $120,000 or so, plus the value of the employer matching. That matching you can perhaps be able to estimate if you look up the benefits section of the website for his employer. Also, even if the 401K provider will not disclose the valuation, the employer's HR department may be willing to disclose his payroll deductions.
In the US it is possible to designate a beneficiary. This is somewhat similar to a joint account here in that the value of the account passes automatically to the named beneficiary without going through probate. It's worth checking if your son made such a designation. Again, the employer may know that if the 401K manager won't reveal.
Then you should factor in the gain in the underlying portfolio which could be estimated from the general return of investments during the elapsed time.
So a very crude estimate might be that the value is as much as $200,000. Based on that I'd say it's worth contacting a US probate lawyer. For a retainer they may be able to get access to the valuation and, on that basis, you can make a more informed decision as to whether it is worth the hassle, stress and cost.
Finally there is a Lemon here called TedSwippet who seems to know a lot about this kind of thing. If he doesn't comment here, PM him.