Hi there, (((hugs))), it's tough to go back to NWs when you're used to sleeping through. I think there could be a coule issues at play. The first is that he's likely going to bed UT. His A times are on the lower side of average and he's still sleeping a decent amount for naps during the day. I would start to gradually increase the A times to 3.5-4hr and also cut the pm nap down to 45min. At this age, I think doing over 2.5hr of day sleep, you can't expect much more than an 11-11.5hr night, so you need to get that BT pushed out a bit.
Secondly, the DF may actually be hindering more than it's helping now. Tracy thought that by 10-12mo, the DF had the potential to actually disrupt sleep by disturbing the sleep cycle more than helping it. You can gradually reduce the amount you give him at the DF to get rid of it, or bring it 15-30min earlier every few nights for a week or two until you drop it completely.
If he truly is in a GS, I'd try to feed him more in the daytime (either a few more oz of milk at each feed) or add in a snack or two, as at this age, they likely don't need it in the MOTN.
What do you think?