Are you estimating 2 hours each way to St Paul's or 2 hours in total per day?
If it's 2 hours each way from Dulwich, to me that would be an absolute NO. Even at 1 hour commute each way, I'd seriously think whether it's necessary - to me the difference between St Paul's and Dulwich College is far too negligible to make an hour's commute necessary. I would think seriously about sitting him for DC in Yr 8 Jan exams if you really want day. Commuting is a major pain (I did 1 1/2 hours each way to school from rural area into town and it made for very limited clubs, social life etc).
We have just made a choice from Eton, Winchester, Westminster and gone with Eton (we were able to dismiss Westminster pretty rapidly whereas the two full boarding options both seemed like the perfect choice ... except there were two for confusion). Part of the reason we ended up with Eton was ease of train journeys there (Putney-Eton = 30 mins direct) and the possibility of meeting for tea, DS1 coming home for Sunday lunch/birthdays etc ... we felt he'd get the benefits of full boarding with more chance to keep family events up too compared with Winchester.
We didn't sit him for Kings or St Paul's as they never seemed like a good fit for him. However DS2 is now in Yr 5 and we're debating schools again. DS2 is keen on day schools. Despite living in Putney, St Paul's is two bus journeys from us and takes 45-50 mins. To me this is a huge negative. It would have to be a great deal better than Kings (10 mins bus, or 15 mins bike) to warrant the difference in commute.
At some point you must have thought that you wanted him to board as you've ended up with two boarding choices and only one day. What's changed?
If you really do feel more pro-day now and don't want a commute, you have within easy reach Whitgift, Trinity, Alleyns (happy with co-ed?), Dulwich College, City of London, all of whom I think test for 13+ for your son's year in January (registrations must be due soon?). Guess you could check them all out?!?
What does your son think? Does he want full boarding?