Hi - well our DD is 27 months old and this happens to us all the time - hee hee! We are experts at lifting Katie out of her car seat, up the stairs and putting her in her bed.
What I would do is not specifically offer him a nap if he doesn't sleep on the way there and back - but keep asking him if he needs a little sleep - if he says yes, maybe you could let him have a quick nap on the couch and wake him after half an hour (before he goes into a deep sleep).
Or, if he falls asleep in the car, either try lifting him into the house still asleep or sitting with him in the car, again for a short nap of no more than half an hour (many a time I have made a cup of tea and brought a book out to read in the car - you could let Jake bring something to read or play with quietly too).
Or, you could just wake him up after a 10 - 15 min catnap in the car and live with a slightly grumpy Harvey for the rest of the day and put him to bed a bit earlier (an hour would be about right).
12 hours sleep overnight would also be a very good sleep for our LO so I would now never worry if she didn't have a nap on those days.
If Harvey does fall asleep mid-play at, say, 4pm (quite possible), you might find it hard to rowse him from his nap and he may be very, very grumpy. What I do in these circumstances is have some food ready early to give as soon as possible after such a late 'catnap', have a quick bath, jammies on and then bed-time routine, by which stage a second wind has probably fallen on the LO!! LOL
Anyway, whatever you decide to do - I hope it works out.
