Oh poor you Cheerful - I went back to work when DS was 9 months old and found it a real heart-wrench so know exactly how you feel.
( I used to phone my DH from the car park in tears for support).
Is it possible that you could change your 'leaving' tactics, its just that you mention he cries 'from the moment he realises you are going to leave' perhaps you could try not let him know when that was going to happen and see if that helps at all.
I read somewhere that you should always say 'goodbye, see you later', as it helps them manage their separation anxiety but think that your DS is probably not quite ready for this yet. As he settles in, you will get to this point naturally.
So sometimes I would 'fly him in' (complete with aeroplane noises all the way up the corridor) then drop a bag and run but would equally sometimes (read more often!) involve me staying for 10-15 mins or so whilst he transferred from me to nursery nurse, then they got him engaged with something?
I believe that this is a difficult time for him, but certainly not that its 'cruel'.
Other things you could try to are to always go in with lots of smiles, talk to him in an excited voice, tell him what is going to happen at every stage, from leaving the house, travelling in car / pushchair / entering the building etc. The repetition of the stages will help settle him into a routine - which babies like.
Ensure that you greet the girls with lots of smiles and excitement - it will all help engender a feeling of security for your DS about the environment you're leaving him in. I don't know if they can really pick up on our stresses, but I worked with the basis that if there was a possibility that it would help him, then it was something I'd do.
I don't know if any of this is even remotely helpful, but I hope things get easier for you both really soon.