ds2 (3.4) and potty training not going well.....advice?

[Becaroooo]

ds1 was 2.10 when dry in the day and took to it like a duck to water (after a couple of false starts!)

ds2 is a whole other ball game Day 1 today (after a couple of false starts last year!) and we have had 3 accidents already.

I am not shouting, being calm, just changing him and getting on with it but am getting really fed up

He will - literally - sit in a dirty nappy for hours and it wont bother him (until his bottom gets red raw and sore of course)

Want him to start pre school after feb half term/easter at latest but want him to be dry for that.

Help????