For me now the main objection is to go swimming.
Especially if there are a few going - I mean has this dad got eyes in the back of his head - is he getting help in the pool?
My Dd who is 5, can swim without armbands, but I've only let her swim with an instructor or me / DH, and GParents (with me on the side of the pool).
They can drown SO quickly. Easy peasy.
And would this dad have help in the change rooms for all the boys who are going - can your DS shampoo his hair himself? Dry himself? dress himself?
And macdonalds - is that supposed to be a treat!!!
I feel that sometimes dads can radically underestimate how much work goes into looking after and making a little person who is far away from home feel at home in a strange house.
IMO 6 is waaay too young to have a sleep over in a strange house, with, in effect, a stranger, after a day of potentially feeling very bereft - if he hasn't mastered his self care techniques yet, he may be getting more and more upset during the day.
To me it sounds potentially extremely miserable for your DS TBH - Good on him that the dad wants to 'treat' his own boy - I think he's being unrealistic about the swimming and the invitation for a sleepover after that is inappropriate.
I don't think you should 'compromise' as someone has posted. This isn't about making this dad's life easier!! This is about your 6 yo son's saftey and happiness.
I think you're right not to let him go at 6.
Plenty of time for all that when he's older.