Your Husband, has NEVER been a part of this. Only you have, for the past NINE years. And your kids, seem fine with it and whatever combinations of kids you have there or not.
Your Husband, COULD take the day off too.
He doesn't.
Or can't.
Or will not.
He can't rule any rules, unless he is a part of this.
Or maybe he feels, that since the eldest son is inviting friends, that therefore.... he is not going to be playing or spending time, with his younger siblings, at all. (as is typical with teens if their friends are around instead). Therefore, it is not, a "family" outing, to your Husband's view of it. Thus, MAYBE.... that is what your Husband is feeling... but is not articulate enough to explain that. Maybe he simply feels, your younger children will not be getting any play time or time at all, with their elder sibling, by themselves. Because his teen friends will be there.
I dunno, that is the only stretch, that I could fathom... per what your Husband may, mean.
Maybe to your Husband, it is either a F.A.M.I.L.Y. day (meaning only family)... or it is just a play-date outing... at the beach. So... maybe to your Husband... WHY IS this, a last-day-of-summer-family-outing-with-your-boys.... if, so many other friends are coming, too? To your Husband, this is probably just a play date at the beach. And it is a mutation... of that "traditional" end-of-summer, tradition, of spending that special last day of summer, with your children. Only.
So to Husband, if you allow one son to invite friends, you should also do the same for ALL your kids and allow them to invite friends. Or do not allow friends at all, and ONLY have that outing with your children. Since that is how the end-of-summer-last-day-of-summer "tradition" initially.... started.
Then, the other problem is: Your son, KNOWING that for the past NINE years, that the last day of summer is ALWAYS spent... with Mom and his siblings, per the tradition you all have. And you even take off of work for it. So, your son, should have known, NOT to make other plans for that SAME day. But he did. He should have asked you FIRST... before confirming his golfing plans, IF he can even go. Before he told his friends, he can. So he.... 'screwed up' the family's last-day-of-summer-outing-plans-tradition... by, telling his friends he would go golfing with them.