I've been told its developmental delay as well and we have definitely seen a certain amount of evidence to back that up, as he can do things now that he couldn't, say, two years ago.
I tend to thing ds's ToM skills are patchy. Sometimes he does seem to get it and others he's not even on the same planet, let alone the same page.
This afternoon I heard him talking to dh "Daddy the dog's come back in the kitchen, he's not supposed to be in here is he, he's supposed to be in the garden at the moment - go on 'dogsname' out, etc etc". I couldn't hear dh, so I stuck my head round the door - sure enough, dh was outside at the other end of the garden and ds could acutally see him through the window! 
He also does the - "look at this picture Mummy" whilst holding the picture towards himself rather than me - thing on a regular basis, as well as talking to me about his book or game as if I know what he knows about the intricacies of the plot/game.
Am I right in thinking those types of things are to do with ToM?
Is the Sally Anne test the one where someone puts an object out of sight while a character is out of the room, then they're asked where the character would think that object is when they re-enter the room and if they say the cupboard, they have ToM problems, because although we know the toy is in the cupboard the character couldn't know because he/she was out of the room? (Sorry wordy but ykwim.)
I ask, because I have a strange feeling ds1 might actually pass that test.
If he did pass it, would that mean he's unlikely to get a dx?
I keep having wobblies about him not being 'bad' enough for a dx. The closer it gets to his Assessment Day (Monday after next) the more I seem to wobble. 