We did some work with him around what emotions different smiley/emoticons represented then started a feelings diary, which to begin with was just three rows of emoticon/smileys on a page. Ds had to circle the one he felt most described however he was feeling at the time he was completing it. He filled this in with his teacher at school, after morning break, after lunch and at the end of the day.
It took a while for him to start to manage it, but when he was consistently being able to identify an emoticon, we moved onto trying to assign an emotion word to the emoticon. It took a long time for him to be able to do it and for the longest time all he would say was, happy, sad or OK, but we did lots of work with him on feelings and emotional language - pointing out when we felt different emotions etc.
Then we added a line for him to explain why he was feeling that emotion.
At that point the Ed Psych got involved and decide it was time to start scaling the emotions. So they set up a scale from 0-10 and assigned what each point on the scale represented eg 0 the worst, 5 OK, 10 Ecstatic. They did this by laying a piece of string out on the carpet and asking him to stand at different points on the rope to represent how he feels when he eg is going to legoland or has to do handwriting practice (two opposite extremes of the emotion scale for my ds 
).
He had to decide at what point on the scale his emotion fell. To begin with we just got 0's or 10's and there was no middle ground - in fact with the way school was back then it was mostly 0's and he would scale the whole day according to one bad thing that upset him. Then very gradually some 5's started to creep in and with a lot of support eventually some more numbers.
From there he has gone on to starting to realise that he feels different emotions throughout the day and the strength or power of his emotions also changes through the day.
We started this when he was just coming up for 8. He's just turned 11 now and has reached the point where he sometimes actually wants to split each entry down, as he recognises eg that maths was horrible so that's a 0 the worst, but then he got some free reading time and that's a 7. Huge progress. 
He now completes the diary first thing in the morning, when he arrives at school, after break, after lunch and at the end of the day. It has helped him to get a handle on his feelings and that not every day is 'the worst ever', but also helps us to gauge his feelings and stress/anxiety levels. The morning one is particularly useful for school to get a baseline of how he is feeling and therefore how the day might proceed, iyswim.
Some people have found this book useful.
and I have also found this feeling and emoticon keyring useful as well.