All my kids are gone by 5pm and we are usually eating our supper by 5:15-5:30. Would you be willing to offer to feed this last child supper for an extra fee? like $3 extra per day? even after I tyoed that i am against that option. I believe its IMPORTANT that your family AND dck's family eat supper together, alone. As a family.
So apart from asking him to be picked up later, essentially changing your daycare closing hours effective Oct 1st or somthing if you want to provide some notice, OR continue doing what your doing and offer him a snack and a quiet toy while you try to enjoy your family dinner.
I personally would not like that, I think 5pm is plenty late enough for me to be open until, and luckily all my clients can work with this too.