OK have read the thread now and everything that you have said (work commitments, not fully involved with the school, living with at your mums, nowhere to call his own space etc) is all the stuff that separated Dads usually have to put up with, even if it was not their choice or their decision to be apart from their child.
All these people saying 'Oh poor you, that is awful, I would hate that.' and this is almost a 50:50 split! Do you ever stop to think how men feel in this situation, when they only get every second weekend? Or do you always assume that the arrangement should chiefly suit the mother, no matter what?
I think you should be grateful your son has a great relationship with both of you, and is able to split his time pretty easily and evenly - there are many children dream of a life like that. There is no law that says everything should always be fine tuned to benefit the mother at the expense of the father, although you'd be forgiven for thinking there was.
You are lucky to be able to live with your mother while you save. I'm sure it's not ideal, but it is doing you a favour. If you wanted an extra night you could always drive over after work and pick him up for a couple of hours. If it's not practical to do that then there's no point worrying over it.
Also, once you've managed to get the deposit for a new place will you be automatically assuming that the situation will change and it can all be rearranged to your liking? Will that still be a pretty even split or will you be expecting it to be weighted much more heavily in your favour? And what do you suppose your DS and your ex will feel about that, when they are quite happy as they are? Will they have a choice? Sorry if I seem to be giving you a hard time but you seem to feel a bit entitled over this, as though its' your right as the mother to have the better end of the deal.
Your son has both parents fully committed to him, and sharing his time pretty equally - that is something to be very grateful for. The rest doesn't matter so long as he is happy. It's just your ego and your pride niggling you. I suspect the arrangement works very well, but you are just concerned about other people's perceptions of you as a mother whose child does not 'live' with her.