Hmmm could be OT..or teething pain? What time did he go for the nap?
If you don't think he will make it to BT, I would try to APOP (car? Buggy?) even just a very, very short (10-15min) CN late afternoon to tide you over to BT. Failing that you just have to push on through with food and distractions to keep him going :-(
Re going to 1 nap, I'd try resettling as long as you can, at least it also gives quiet time. They do get the hang of it eventually but it is an unsettled time. Lots of LOs the same age as yours do well on set naps... What was your naptime the last couple of days and maybe we can think of a set routine to try?
ETA- just seen your post on 2-1 chat thread & also replied there but I think given late transition to 2 naps, we might be pushing to 1 nap too soon & maybe he does need the late pm micro nap that we have talked about before - 4,30 or 4.45 for 15mins could still give you a 7pm BT.
Xxx