How can you tap into that for a wedding? Assuming he'll be in his 40s in 20 years (just had his first kid)
There can be very complicated accounts and everyone needs to know their situation well but perhaps:
If they're gonna be 59.5 then they could.
Roth IRA principal withdrawals are allowed anytime - then the growth can be left for later weddings/retirement/etc.
I think some 401K's allow borrowing for a small fee and the "interest" is going back to yourself.
HSA I think if you can show historical medical expenses you can withdraw.
Again these can be complicated vehicles but there can be significant advantages that should be looked at.
In general, you'd pay long-term capital gains, which is generally lower than regular income tax.
I think 15% for married joint from ~80-500K income this year.
ETA I think cap gain 0% for under 80K income married joint.