You were very young when you got together, he was a teenager, you several years away from full maturity.
You said he was shy and vanilla - he's now realising what he thinks he missed, the wild-oats-sowing phase, the growing up, getting to know himself, having adventures and experiences he thinks other men are having all over the place.
He never experienced sex with different women, different tastes and practices.
He thinks he's missed out.
He's a bit young for a midlife crisis, but he launched straight into a relationship as as soon as he hit adulthood, followed by fatherhood.
This hippy friend - is he definitely just a friend? No possibility that your DH has realised there's something else he missed out on?
It's also quite possible he's had some adventures on his trips away for work and solo holidays. What happens on location stays on location, as they say.
Time for a calm adult conversation about how he and you both see the relationship, the marriage, the family, the future looking. Are you actually compatible, in your adult forms?
I'd almost say let him off the leash for a bit for a runaround, make being careful a condition. See if he wants to come back and if yes, to take an STI test. He may come back ready to settle down.
You on the other hand may decide the fact he even accepted the offer is the end.
If you separate he'll have to step up and pay up - 50:50 in time, childcare and cash; which he seems to have ducked out of quite successfully so far. He might find separating and living the single life would tie him down far more than when you were around shouldering the wife and care work.