My dd was clingy from birth with major separation anxiety setting in at 5 months. By 9 months, it was slightly better and she would let herself be carried by grandma for 30 minutes (happened once!). At 2.5 years, she reliably will notice when I leave a room within 30 seconds, and will start to call for me and come and get me if I don't respond or even if I reassure that I am just going to another room for a while. Yes, it will escalate to crying if I don't come back but it does not usually get to that stage.
She goes to nursery fulltime since 18 months, building up from 2 morning sessions at 13 months. She has periods of crying at the drop off and periods where she happily goes to carers with a kiss for mummy. But generally, she settles within minutes once I am gone.
snowleopard, check with the carers how long ds cries for after you are gone and how he is generally in nursery. Just a matter-of-fact kiss goodbye and smile is all you need before you leave the room. It breaks your heart to see them cry but it is very likely he is fine once you are gone.
As for clinginess, in my dd, it was more than a phase, it is part of her make up. She is just generally a people person and very attached to mummy and daddy. At home, I go to her most of the time, even if it means I have to work around her to get any work done around the house (usually very little once I have picked her up from nursery). I have no fears of spoiling her - from crying hysterically when I leave the room, she is now calm and starts with "mummeeee" before looking for me and grabbing me by my hand to pull me back to her.
I don't believe in all this supernanny business or that I am being soft. As a parent, I am there to meet dd's needs. Once he is ready to go into his own world, I have no doubt your ds would get over his separation anxiety. In the meantime, reassuring him with your presence (at home) will give him the confidence to move on sooner. 