There is something else I think besides the great advice above (often towns and cities have something I know NYC does, they have retired business people who are there to help those starting a business or thinking about starting a business, can be really valuable).
The biggest thing I think that the OP and their spouse can do is to encourage the kid, and also to let them know that as much as you can, they have something of a net available if things don’t go right. If the kid is truly entrepeneurial, knowing that someone has his back, even if it is simply to give them encouragement, is huge.
They will have plenty of people telling them that going the entrepeneurial route is impossible, that they likely will fail, that it is the road to ruin, that they should be looking for a job that pays well and will provide career advancement, benefits, etc. He needs to hear that it is okay to fail, that typical entrepeneurs who are successful have quite a few failures before becoming successful, that trying and failing is not a crime nor does it reflect on him. One of the things that scares me these days with parent’s obsession about kids being on the right path to an elite school, the whole focus on doing things that are safe or play the admissions game, is that that is the direct opposite of entrepeneurial spirit, the whole idea is to do something new and different and take risks. And in this climate, many of the same people who laud business people who have created something new are often the same people saying ‘you’ll be sorry’, yet those business people failed a lot (I think the typical profile is like 6 failures before a success).
Having his parents encourage him will go a long way to dispel the naysayers, as much as we think we don’t have influence, we do. In some ways it isn’t much different than kids going into the arts, which is a leap that is seen as risky and potentially fatal, and encouragement and support is huge. Again, knowing there is someone there encouraging them and also who has their back is huge, even if the only thing parents can give is their love and support, that is huge. Thing is, if the kid is entrepeneurial at heart, then he will want to do it on his own and likely only needs the encouragement (that, and maybe keeping him on your health insurance until he is 26:).
Another thought is he should reach out to entrepeneurs he has heard of, write to them, and ask for advice, I know two people who did very well as entrepeneurs, the guy who is the CEO of the company I work for now, and the guy who founded our division and sold it to this company, and both of them love it when others reach out to them and ask for advice, because they were there and know what it is like:).