Even if dad is spending time with his son there is no reason why some of that time can't be sharing lunch together so having a rule that if he comes to care after 12:30 he must be fed. To get lunch he must come before 12:00 ie you are all eating at the same time and child is either eating at home or your place depending on drop off time. Then there is a planned playtime after followed by nap. If child comes fed he is ready to play anyways.

Alternately for the sake of your own child and future children who will come having eaten breakfast before 7 am and want lunch by noon you are going to want to have a 12:00 lunch every day as you fill up so while this works for now you are going to want to back it up later. Might be better to do it now. 12:00 is lunch and I feed whoever is here. Meal of the day is only guaranteed if I know by 11:30 you are coming and NO you may not send your child lunch in hand as that isnt' fair to the rest of the gang.