Would it be better to split his investments between iDeCo and NISA?

[Beaglehound]

I think the strategy you have outlined is good. He will benefit from tax savings now with the IDECO portion while putting the rest in the more flexible NISA.

At his age, everything in All Country is also perfectly reasonable.

One caveat is that you did mention in the OP that he may want to withdraw the money. If that is a serious possibility, then 100% stocks is problematic. He may want to build up more of a cash fund to prevent the possibility of having to withdraw at a loss.

[Oliver170]

By this you mean, having 3 to six months of savings for expenses? And / Or also just having some (savings) money in the bank for life opportunities?

[Beaglehound]

By this you mean, having 3 to six months of savings for expenses? And / Or also just having some (savings) money in the bank for life opportunities?

Yes to the first. And if, for example, there is a possibility of buying a house in the next few years, yes to the second as well. In general it is very risky to be putting money in the stock market that you may need in the next few years.

[Tkydon]

1. **Allocation:** Should he split his investments between iDeCo (¥20,000) and NISA (¥40,000)?

This would total in the year ¥60,000 x 12 = ¥720,000 Invested, whether exclusively NISA or iDeCo/NISA combination...

If he does this, he'll have a tax advantage as I showed above of a Year-End Refund of National Tax, and a reduction of Residential Tax on that year's income from the following June...

So, roughly, he should be able to add the Tax Refund to NISA at the end of the year

Assuming a Full Tax Year's iDeCo Contributions - 12 months from Jan to Dec

iDeCo (¥20,000 x 12 = ¥240,000)

and NISA (¥40,000 x 12 = ¥480,000 + say 25% of the Y240,000 iDeCo National Tax refund in Dec Year End Adjustment Nenmatsu Chousei (¥60,000)
Total ¥540,000

Year Total = ¥240,000 + ¥540,000 = ¥780,000, up from ¥720,000

and in the following year, from June up the NISA Monthly payment

iDeCo (¥20,000 x 12 = ¥240,000 - no change)

and NISA (¥40,000 + say 13% of ¥240,000 Residents' Tax Reduction / 12 = additional ¥2,600 per month,
= ¥40,000 per month x 5 + ¥42,600 per month x 7 = ¥498,200 for the year, up from ¥480,000
and say 25% of the Y240,000 National Tax refund (¥60,000)
Total ¥558,200 in the second year

Year Total = ¥240,000 + ¥558,200 = ¥798,200, up from ¥720,000

third year and beyond
and NISA (¥40,000 + say 13% of ¥240,000 Residents' Tax Reduction / 12 = additional ¥2,600 per month,
= ¥42,600 per month x 12 = ¥511,200 per year, up from ¥480,000
and say 25% of the Y240,000 National Tax refund (¥60,000)
Total ¥571,200 per year

Year Total = ¥240,000 + ¥571,200 = ¥811,200, up from ¥720,000

with absolutely no additional cost to himself, all funded by the tax savings.

The iDeCo will be taxable in retirement, but the NISA will be completely Tax Free, funded partially by the Tax Free Tax savings.

Compared to exclusive NISA, his NISA porttion is still lower, but the total invested is significantly up, and I just worked that on the lower tax savings I calculated above. If you work on the next tax band marginal tax rate then the total benefit would be greater...
And he can withdraw from NISA at any time.