General advice/tips please for 3.5yr old

[ladypop]

Our 3.5yr old DS is yet to be toilet trained. We tried a few times throughout his 2nd year but to no avail and I had a baby 2 months ago so decided to wait until now rather than attempt so close to birth.
Well, the time is now and I don't really know where to start! maybe i try and blitz it ie, nappy off, stay at home and really go for it over the course of a few days. however, he goes to nursery 2 days a week as well as grandparents another day. How do your manage this kind of approach when you actually need them to be elsewhere and it isn't possible to leave them half naked (ie nursery)? ....just not sure what tactic to take!
He is just not bothered, is happy to have his nappy changed and doesn't seem bothered that most of his friends are all wearing 'big boy pants' etc. but time is ticking and I do want to crack it this time whilst I am on maternity leave before I return to work on sept......general advice needed please!!