Ds is no good if asked why he is sad (he does better with happy e.g. 'because tickling' -meaning he likes being tickled)
Does your DS properly understand why/because? DS really struggles with this even in simple sentences e.g. show him a picture of child with umbrella and say 'why has he got umbrella' - DS can say 'because its raining'. But there was a SLT picture of a boy crying because a dog was chewing his shoe and DS could not link the boy crying to the shoe.
You may need to explore using comprehension pictures to be sure he really understands the concept of why/because. DS has worked on this for a while but still hasn't really got it.
DS is much better at saying like / dislike (more concrete - as only 2 choices, with 'why' there can be lots of possible answers).
We are trying to get DS to choose his own rewards more - so teaching language of 'like, don't like, ok, favourite'. Often DS won't say what he likes just look around the room, see something and say that just to say something! It can be that pressure to use language that causes added stress. So sometimes we use sorting to get him to show us what he likes most, whats ok, what he doesn't like by putting three pieces paper on floor with words written on them and DS has to sort items into three piles.
He's much better at saying what he does not like!
We're also working on teaching him to recognise when he is cross and relaxed (doing relaxation techniques), so he can calm himself down. (Its quite funny as he will now say 'I'm cross, deep breathing (then blows out hard!). Even just that self awareness is huge for DS. Before we started working on it he could not really identify his own level of stress. We also videoed him being excited and cross and showed them to him so he could label those emotions in himself - so we would show him the video and say are you happy, excited, cross etc - it was amazing how he couldn't often give the right label.
So now when we ask him to do something he doesn't want to (turning down volume on iPad is key one at moment), he will say 'I'm cross, don't like turn it down'. Sometimes he will write this down rather than say it.
Why does your DS have to say if he is happy or sad and why - isn't it obvious if he is or isn't?
He might find like / dislike easier and start with sorting say a list of activities into piles. Then he may like my DS start using that language during the day for things he doesn't like (which tend to be what makes him unhappy)