I just transitioned two 12mo olds and one was easy, the other took 6 weeks! We just turned the corner last week, he has really settled in, but the first few weeks were hell. lol. I let him cry for up to an hour at nap time, and eventually he realized the routine and now is asleep in minutes. I had told his parents that this is how naps will be handled and they were fine with it.
He also cried or fussed most of the day - just wasn't used to me, my home or the other kids. The first 2 weeks I kept him close to me, and if I wasn't down on the floor with the kids I put him in his highchair, he felt safe in there. Gradually I held him less and less, and he would venture further on his own and come to "check in" with me every min or so. Today I realized how much progress he's made when I was watching him right in the thick of things with the other kids and having a great time
Something I always do, is tell the parents exactly what is happening during transition. So I told them about all the crying, the not sleeping, etc, and also that I felt it was normal at this stage. That way, if progress wasn't being made, it wouldn't be a shocker to them that I give notice.
Good luck, and hang in there. I was really ready to give up with this guy, as all my other 12mo olds transitioned easily, but I think he is going to end up being the most easy-going of my bunch!