to be honest I don't think you have much choice!
I agree, there isn't much choice. He will have to hang on for his nap until he gets home.
One thing you might be able to do is shift "lunch" much earlier. If he is used to napping at 1pm but you are not even getting home until say 1.30pm he might be too tired to entertain lunch and need to go directly for his nap immediately. sometimes LO can only cope with these outings if they are allowed to go right to sleep at the first opportunity and with no delay.
You might find you can give a substantial "snack" before heading out to play school. This will help him have more energy to cope with the class and to last a bit longer before the nap. Although it might seem very early offering a lunch type meal at 10.30 or 11am it may help his mood. You could then do another substantial "snack" when he wakes at 3.30/4pm and still offer dinner at your usual time.
If he is safe to eat during the journey home, finger foods, that might help too.
Many of my DS's toddler groups I took lunch type finger food snacks such as mini bean burgers or falafel so that he got more of a lunch type meal. He could really eat a lot at snack time.